[clang] [clang-tools-extra] [clang-tidy] Add new check `modernize-use-designated-initializers` (PR #80541)

[Danny Mösch via cfe-commits]

https://github.com/SimplyDanny updated https://github.com/llvm/llvm-project/pull/80541

>From 528ec390e23e6883356606c9acdba3315d5f59bb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 13:13:50 +0100
Subject: [PATCH 01/23] Trigger on variable declarations

---
 .../clang-tidy/modernize/CMakeLists.txt       |  1 +
 .../modernize/ModernizeTidyModule.cpp         |  3 +
 .../UseDesignatedInitializersCheck.cpp        | 58 +++++++++++++++++++
 .../UseDesignatedInitializersCheck.h          | 33 +++++++++++
 clang-tools-extra/docs/ReleaseNotes.rst       |  4 ++
 .../docs/clang-tidy/checks/list.rst           |  1 +
 .../modernize/use-designated-initializers.rst |  6 ++
 .../modernize/use-designated-initializers.cpp | 32 ++++++++++
 8 files changed, 138 insertions(+)
 create mode 100644 clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
 create mode 100644 clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
 create mode 100644 clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
 create mode 100644 clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp

diff --git a/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt b/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
index 28ca52f46943a8..6852db6c2ee311 100644
--- a/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
@@ -31,6 +31,7 @@ add_clang_library(clangTidyModernizeModule
   UseBoolLiteralsCheck.cpp
   UseConstraintsCheck.cpp
   UseDefaultMemberInitCheck.cpp
+  UseDesignatedInitializersCheck.cpp
   UseEmplaceCheck.cpp
   UseEqualsDefaultCheck.cpp
   UseEqualsDeleteCheck.cpp
diff --git a/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp b/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
index 654f4bd0c6ba47..e96cf274f58cfe 100644
--- a/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
@@ -32,6 +32,7 @@
 #include "UseBoolLiteralsCheck.h"
 #include "UseConstraintsCheck.h"
 #include "UseDefaultMemberInitCheck.h"
+#include "UseDesignatedInitializersCheck.h"
 #include "UseEmplaceCheck.h"
 #include "UseEqualsDefaultCheck.h"
 #include "UseEqualsDeleteCheck.h"
@@ -68,6 +69,8 @@ class ModernizeModule : public ClangTidyModule {
     CheckFactories.registerCheck<MakeSharedCheck>("modernize-make-shared");
     CheckFactories.registerCheck<MakeUniqueCheck>("modernize-make-unique");
     CheckFactories.registerCheck<PassByValueCheck>("modernize-pass-by-value");
+    CheckFactories.registerCheck<UseDesignatedInitializersCheck>(
+        "modernize-use-designated-initializers");
     CheckFactories.registerCheck<UseStartsEndsWithCheck>(
         "modernize-use-starts-ends-with");
     CheckFactories.registerCheck<UseStdNumbersCheck>(
diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
new file mode 100644
index 00000000000000..06f0bb0dc06eac
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -0,0 +1,58 @@
+//===--- UseDesignatedInitializersCheck.cpp - clang-tidy ------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "UseDesignatedInitializersCheck.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/Stmt.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include <algorithm>
+#include <iterator>
+#include <vector>
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::modernize {
+
+static std::vector<Stmt *>
+getAllUndesignatedInits(const InitListExpr *SyntacticInitList) {
+  std::vector<Stmt *> Result;
+  std::copy_if(SyntacticInitList->begin(), SyntacticInitList->end(),
+               std::back_inserter(Result),
+               [](auto S) { return !isa<DesignatedInitExpr>(S); });
+  return Result;
+}
+
+void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(varDecl(allOf(has(initListExpr().bind("init")),
+                                   hasType(recordDecl().bind("type")))),
+                     this);
+}
+
+void UseDesignatedInitializersCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *InitList = Result.Nodes.getNodeAs<InitListExpr>("init");
+  const auto *Type = Result.Nodes.getNodeAs<CXXRecordDecl>("type");
+  if (!Type || !InitList || !Type->isAggregate())
+    return;
+  if (const auto *SyntacticInitList = InitList->getSyntacticForm()) {
+    const auto UndesignatedParts = getAllUndesignatedInits(SyntacticInitList);
+    if (UndesignatedParts.empty())
+      return;
+    if (UndesignatedParts.size() == SyntacticInitList->getNumInits()) {
+      diag(InitList->getLBraceLoc(), "use designated initializer list");
+      return;
+    }
+    for (const auto *InitExpr : UndesignatedParts) {
+      diag(InitExpr->getBeginLoc(), "use designated init expression");
+    }
+  }
+}
+
+} // namespace clang::tidy::modernize
diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
new file mode 100644
index 00000000000000..aeaa3f19deb7e8
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
@@ -0,0 +1,33 @@
+//===--- UseDesignatedInitializersCheck.h - clang-tidy ----------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USEDESIGNATEDINITIALIZERSCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USEDESIGNATEDINITIALIZERSCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::modernize {
+
+/// FIXME: Write a short description.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/modernize/use-designated-initializers.html
+class UseDesignatedInitializersCheck : public ClangTidyCheck {
+public:
+  UseDesignatedInitializersCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+  std::optional<TraversalKind> getCheckTraversalKind() const override {
+    return TK_IgnoreUnlessSpelledInSource;
+  }
+};
+
+} // namespace clang::tidy::modernize
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USEDESIGNATEDINITIALIZERSCHECK_H
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index ee68c8f49b3df2..4ee1af44b9b37d 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -105,6 +105,10 @@ New checks
 
   Replaces certain conditional statements with equivalent calls to
   ``std::min`` or ``std::max``.
+- New :doc:`modernize-use-designated-initializers
+  <clang-tidy/checks/modernize/use-designated-initializers>` check.
+
+  FIXME: add release notes.
 
 New check aliases
 ^^^^^^^^^^^^^^^^^
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index 59ef69f390ee91..5e57bc0ee483fe 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -287,6 +287,7 @@ Clang-Tidy Checks
    :doc:`modernize-use-bool-literals <modernize/use-bool-literals>`, "Yes"
    :doc:`modernize-use-constraints <modernize/use-constraints>`, "Yes"
    :doc:`modernize-use-default-member-init <modernize/use-default-member-init>`, "Yes"
+   :doc:`modernize-use-designated-initializers <modernize/use-designated-initializers>`, "Yes"
    :doc:`modernize-use-emplace <modernize/use-emplace>`, "Yes"
    :doc:`modernize-use-equals-default <modernize/use-equals-default>`, "Yes"
    :doc:`modernize-use-equals-delete <modernize/use-equals-delete>`, "Yes"
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
new file mode 100644
index 00000000000000..e0487f9b0e843d
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
@@ -0,0 +1,6 @@
+.. title:: clang-tidy - modernize-use-designated-initializers
+
+modernize-use-designated-initializers
+=====================================
+
+FIXME: Describe what patterns does the check detect and why. Give examples.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
new file mode 100644
index 00000000000000..c945454201ee13
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -0,0 +1,32 @@
+// RUN: %check_clang_tidy %s modernize-use-designated-initializers %t
+
+struct S1 {};
+
+S1 s11{};
+S1 s12 = {};
+S1 s13();
+S1 s14;
+
+struct S2 { int i, j; };
+
+S2 s21{.i=1, .j =2};
+
+S2 s22 = {1, 2};
+// CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
+
+S2 s23{1};
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use designated initializer list [modernize-use-designated-initializers]
+
+S2 s24{.i = 1};
+
+S2 s25 = {.i=1, 2};
+// CHECK-MESSAGES: :[[@LINE-1]]:17: warning: use designated init expression [modernize-use-designated-initializers]
+
+struct S3 {
+    S2 s2;
+    double d;
+};
+
+S3 s31 = {.s2 = 1, 2, 3.1};
+// CHECK-MESSAGES: :[[@LINE-1]]:20: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-MESSAGES: :[[@LINE-2]]:23: warning: use designated init expression [modernize-use-designated-initializers]

>From ca32ce75d155ca17e8dc237d41047b38afd68f9c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 13:34:58 +0100
Subject: [PATCH 02/23] Support nested initializer lists

---
 .../modernize/UseDesignatedInitializersCheck.cpp     |  5 ++---
 .../modernize/use-designated-initializers.cpp        | 12 ++++++++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index 06f0bb0dc06eac..e0400db473db25 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -30,9 +30,8 @@ getAllUndesignatedInits(const InitListExpr *SyntacticInitList) {
 }
 
 void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
-  Finder->addMatcher(varDecl(allOf(has(initListExpr().bind("init")),
-                                   hasType(recordDecl().bind("type")))),
-                     this);
+  Finder->addMatcher(
+      initListExpr(hasType(recordDecl().bind("type"))).bind("init"), this);
 }
 
 void UseDesignatedInitializersCheck::check(
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index c945454201ee13..a5eeb5553a9093 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -30,3 +30,15 @@ struct S3 {
 S3 s31 = {.s2 = 1, 2, 3.1};
 // CHECK-MESSAGES: :[[@LINE-1]]:20: warning: use designated init expression [modernize-use-designated-initializers]
 // CHECK-MESSAGES: :[[@LINE-2]]:23: warning: use designated init expression [modernize-use-designated-initializers]
+
+S3 s32 = {{.i = 1, 2}};
+// CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-MESSAGES: :[[@LINE-2]]:20: warning: use designated init expression [modernize-use-designated-initializers]
+
+struct S4 {
+    double d;
+    private: static int i;
+};
+
+S4 s41 {2.2};
+// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: use designated initializer list [modernize-use-designated-initializers]

>From 1675bcf2bbc821cbe41f7caec64e8251a11abf51 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 13:35:12 +0100
Subject: [PATCH 03/23] Rename

---
 .../modernize/UseDesignatedInitializersCheck.cpp      | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index e0400db473db25..7cf6dcb21ac1be 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -21,7 +21,7 @@ using namespace clang::ast_matchers;
 namespace clang::tidy::modernize {
 
 static std::vector<Stmt *>
-getAllUndesignatedInits(const InitListExpr *SyntacticInitList) {
+getUndesignatedComponents(const InitListExpr *SyntacticInitList) {
   std::vector<Stmt *> Result;
   std::copy_if(SyntacticInitList->begin(), SyntacticInitList->end(),
                std::back_inserter(Result),
@@ -41,14 +41,15 @@ void UseDesignatedInitializersCheck::check(
   if (!Type || !InitList || !Type->isAggregate())
     return;
   if (const auto *SyntacticInitList = InitList->getSyntacticForm()) {
-    const auto UndesignatedParts = getAllUndesignatedInits(SyntacticInitList);
-    if (UndesignatedParts.empty())
+    const auto UndesignatedComponents =
+        getUndesignatedComponents(SyntacticInitList);
+    if (UndesignatedComponents.empty())
       return;
-    if (UndesignatedParts.size() == SyntacticInitList->getNumInits()) {
+    if (UndesignatedComponents.size() == SyntacticInitList->getNumInits()) {
       diag(InitList->getLBraceLoc(), "use designated initializer list");
       return;
     }
-    for (const auto *InitExpr : UndesignatedParts) {
+    for (const auto *InitExpr : UndesignatedComponents) {
       diag(InitExpr->getBeginLoc(), "use designated init expression");
     }
   }

>From b6a8e57dee25b69bb3a30f952e6eebb5829ddd45 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 13:36:10 +0100
Subject: [PATCH 04/23] Enable for C++20 only

---
 .../clang-tidy/modernize/UseDesignatedInitializersCheck.h      | 3 +++
 .../checkers/modernize/use-designated-initializers.cpp         | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
index aeaa3f19deb7e8..12a9850ada6a14 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
@@ -26,6 +26,9 @@ class UseDesignatedInitializersCheck : public ClangTidyCheck {
   std::optional<TraversalKind> getCheckTraversalKind() const override {
     return TK_IgnoreUnlessSpelledInSource;
   }
+  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+    return LangOpts.CPlusPlus20;
+  }
 };
 
 } // namespace clang::tidy::modernize
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index a5eeb5553a9093..7e41943e884061 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -1,4 +1,4 @@
-// RUN: %check_clang_tidy %s modernize-use-designated-initializers %t
+// RUN: %check_clang_tidy -std=c++20 %s modernize-use-designated-initializers %t
 
 struct S1 {};
 

>From b0663cd95a21ec49f09ad339a6086eda9296b800 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 13:56:08 +0100
Subject: [PATCH 05/23] Add option to ignore aggregates with single elements

---
 .../UseDesignatedInitializersCheck.cpp        | 19 +++++++++++++++++++
 .../UseDesignatedInitializersCheck.h          |  9 +++++++--
 .../modernize/use-designated-initializers.cpp |  5 ++++-
 3 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index 7cf6dcb21ac1be..d269cef13e6aaa 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -20,6 +20,10 @@ using namespace clang::ast_matchers;
 
 namespace clang::tidy::modernize {
 
+static constexpr auto IgnoreSingleElementAggregatesName =
+    "IgnoreSingleElementAggregates";
+static constexpr auto IgnoreSingleElementAggregatesDefault = true;
+
 static std::vector<Stmt *>
 getUndesignatedComponents(const InitListExpr *SyntacticInitList) {
   std::vector<Stmt *> Result;
@@ -29,6 +33,13 @@ getUndesignatedComponents(const InitListExpr *SyntacticInitList) {
   return Result;
 }
 
+UseDesignatedInitializersCheck::UseDesignatedInitializersCheck(
+    StringRef Name, ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context),
+      IgnoreSingleElementAggregates(
+          Options.getLocalOrGlobal(IgnoreSingleElementAggregatesName,
+                                   IgnoreSingleElementAggregatesDefault)) {}
+
 void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
       initListExpr(hasType(recordDecl().bind("type"))).bind("init"), this);
@@ -40,6 +51,8 @@ void UseDesignatedInitializersCheck::check(
   const auto *Type = Result.Nodes.getNodeAs<CXXRecordDecl>("type");
   if (!Type || !InitList || !Type->isAggregate())
     return;
+  if (IgnoreSingleElementAggregates && InitList->getNumInits() == 1)
+    return;
   if (const auto *SyntacticInitList = InitList->getSyntacticForm()) {
     const auto UndesignatedComponents =
         getUndesignatedComponents(SyntacticInitList);
@@ -55,4 +68,10 @@ void UseDesignatedInitializersCheck::check(
   }
 }
 
+void UseDesignatedInitializersCheck::storeOptions(
+    ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, IgnoreSingleElementAggregatesName,
+                IgnoreSingleElementAggregates);
+}
+
 } // namespace clang::tidy::modernize
diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
index 12a9850ada6a14..8a60fd7b31c5a4 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
@@ -19,16 +19,21 @@ namespace clang::tidy::modernize {
 /// http://clang.llvm.org/extra/clang-tidy/checks/modernize/use-designated-initializers.html
 class UseDesignatedInitializersCheck : public ClangTidyCheck {
 public:
-  UseDesignatedInitializersCheck(StringRef Name, ClangTidyContext *Context)
-      : ClangTidyCheck(Name, Context) {}
+  UseDesignatedInitializersCheck(StringRef Name, ClangTidyContext *Context);
   void registerMatchers(ast_matchers::MatchFinder *Finder) override;
   void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
   std::optional<TraversalKind> getCheckTraversalKind() const override {
     return TK_IgnoreUnlessSpelledInSource;
   }
   bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
     return LangOpts.CPlusPlus20;
   }
+  void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
+
+private:
+  bool IgnoreSingleElementAggregates;
 };
 
 } // namespace clang::tidy::modernize
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index 7e41943e884061..350ee03d581ae8 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -1,4 +1,7 @@
 // RUN: %check_clang_tidy -std=c++20 %s modernize-use-designated-initializers %t
+// RUN: %check_clang_tidy -check-suffixes=,SINGLE-ELEMENT -std=c++20 %s modernize-use-designated-initializers %t \
+// RUN:     -- -config="{CheckOptions: [{key: modernize-use-designated-initializers.IgnoreSingleElementAggregates, value: false}]}" \
+// RUN:     --
 
 struct S1 {};
 
@@ -41,4 +44,4 @@ struct S4 {
 };
 
 S4 s41 {2.2};
-// CHECK-MESSAGES: :[[@LINE-1]]:8: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-MESSAGES-SINGLE-ELEMENT: :[[@LINE-1]]:8: warning: use designated initializer list [modernize-use-designated-initializers]

>From 4aee8852b4c0ce145b2ffbceb33a8b39238d0ff4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 14:06:17 +0100
Subject: [PATCH 06/23] Test class

---
 .../checkers/modernize/use-designated-initializers.cpp         | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index 350ee03d581ae8..d932e2d5e4863b 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -25,7 +25,8 @@ S2 s24{.i = 1};
 S2 s25 = {.i=1, 2};
 // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: use designated init expression [modernize-use-designated-initializers]
 
-struct S3 {
+class S3 {
+  public:
     S2 s2;
     double d;
 };

>From 4a0679beccdfe1956074607815e5841ddaf1d8ee Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 14:22:56 +0100
Subject: [PATCH 07/23] Add documentation and release notes

---
 .../UseDesignatedInitializersCheck.h          |  3 +-
 clang-tools-extra/docs/ReleaseNotes.rst       |  3 +-
 .../modernize/use-designated-initializers.rst | 40 ++++++++++++++++++-
 3 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
index 8a60fd7b31c5a4..34290aba06fab1 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
@@ -13,7 +13,8 @@
 
 namespace clang::tidy::modernize {
 
-/// FIXME: Write a short description.
+/// Triggers on initializer lists for aggregate type that could be
+/// written as designated initializers instead.
 ///
 /// For the user-facing documentation see:
 /// http://clang.llvm.org/extra/clang-tidy/checks/modernize/use-designated-initializers.html
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 4ee1af44b9b37d..2e3a2027cedc38 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -108,7 +108,8 @@ New checks
 - New :doc:`modernize-use-designated-initializers
   <clang-tidy/checks/modernize/use-designated-initializers>` check.
 
-  FIXME: add release notes.
+  Triggers on initializer lists for aggregate type that could be
+  written as designated initializers instead.
 
 New check aliases
 ^^^^^^^^^^^^^^^^^
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
index e0487f9b0e843d..1e182218dbd8eb 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
@@ -3,4 +3,42 @@
 modernize-use-designated-initializers
 =====================================
 
-FIXME: Describe what patterns does the check detect and why. Give examples.
+Triggers on initializer lists for aggregate type that could be written as
+designated initializers instead.
+
+With plain initializer lists, it is very easy to introduce bugs when adding
+new fields in the middle of a struct or class type. The same confusion might
+arise when changing the order of fields.
+
+C++ 20 supports the designated initializer syntax for aggregate types.
+By applying it, we can always be sure that aggregates are constructed correctly,
+because the every variable being initialized is referenced by name.
+
+Example:
+
+.. code-block::
+
+    struct S { int i, j; };
+
+is an aggregate type that should be initialized as
+
+.. code-block::
+
+    S s{.i = 1, .j = 2};
+
+instead of
+
+.. code-block::
+
+    S s{1, 2};
+
+which could easily become an issue when ``i`` and ``j`` are swapped in the
+declaration of ``S``.
+
+Options
+-------
+
+.. options:: IgnoreSingleElementAggregates
+
+    The value `false` specifies that even initializers for aggregate types
+    with only a single element should be checked. The default value is `true`.

>From 145c6e5385bd5107b053c6d99cf10ca47eaae203 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 14:31:30 +0100
Subject: [PATCH 08/23] Fix typo

---
 .../clang-tidy/checks/modernize/use-designated-initializers.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
index 1e182218dbd8eb..27474921a53d5e 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
@@ -38,7 +38,7 @@ declaration of ``S``.
 Options
 -------
 
-.. options:: IgnoreSingleElementAggregates
+.. option:: IgnoreSingleElementAggregates
 
     The value `false` specifies that even initializers for aggregate types
     with only a single element should be checked. The default value is `true`.

>From bcb2da505a8f3c77f82bf7ddd3fbf48f749aac7f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 17:24:32 +0100
Subject: [PATCH 09/23] Specify explicit type

---
 .../clang-tidy/modernize/UseDesignatedInitializersCheck.cpp   | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index d269cef13e6aaa..7b23a2e8a09308 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -20,9 +20,9 @@ using namespace clang::ast_matchers;
 
 namespace clang::tidy::modernize {
 
-static constexpr auto IgnoreSingleElementAggregatesName =
+static const char *IgnoreSingleElementAggregatesName =
     "IgnoreSingleElementAggregates";
-static constexpr auto IgnoreSingleElementAggregatesDefault = true;
+static const bool IgnoreSingleElementAggregatesDefault = true;
 
 static std::vector<Stmt *>
 getUndesignatedComponents(const InitListExpr *SyntacticInitList) {

>From 4caec58c15f9eafdf80d49e1d5e6457574f8cb7b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 17:24:44 +0100
Subject: [PATCH 10/23] Get only local option

---
 .../clang-tidy/modernize/UseDesignatedInitializersCheck.cpp   | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index 7b23a2e8a09308..0594f3dfb10306 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -37,8 +37,8 @@ UseDesignatedInitializersCheck::UseDesignatedInitializersCheck(
     StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),
       IgnoreSingleElementAggregates(
-          Options.getLocalOrGlobal(IgnoreSingleElementAggregatesName,
-                                   IgnoreSingleElementAggregatesDefault)) {}
+          Options.get(IgnoreSingleElementAggregatesName,
+                      IgnoreSingleElementAggregatesDefault)) {}
 
 void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(

>From 909d2bbfa5b019a884a4955bac8aafb41adc3ff7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 17:25:03 +0100
Subject: [PATCH 11/23] Improve check description

---
 .../clang-tidy/modernize/UseDesignatedInitializersCheck.h       | 2 +-
 clang-tools-extra/docs/ReleaseNotes.rst                         | 2 +-
 .../clang-tidy/checks/modernize/use-designated-initializers.rst | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
index 34290aba06fab1..de06250b0d3d06 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
@@ -13,7 +13,7 @@
 
 namespace clang::tidy::modernize {
 
-/// Triggers on initializer lists for aggregate type that could be
+/// Finds initializer lists for aggregate type that could be
 /// written as designated initializers instead.
 ///
 /// For the user-facing documentation see:
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 2e3a2027cedc38..3effe9f9d5b5f0 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -108,7 +108,7 @@ New checks
 - New :doc:`modernize-use-designated-initializers
   <clang-tidy/checks/modernize/use-designated-initializers>` check.
 
-  Triggers on initializer lists for aggregate type that could be
+  Finds initializer lists for aggregate type that could be
   written as designated initializers instead.
 
 New check aliases
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
index 27474921a53d5e..0f313eec2b1990 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
@@ -3,7 +3,7 @@
 modernize-use-designated-initializers
 =====================================
 
-Triggers on initializer lists for aggregate type that could be written as
+Finds initializer lists for aggregate type that could be written as
 designated initializers instead.
 
 With plain initializer lists, it is very easy to introduce bugs when adding

>From 91bf1b6b48323c454f3f10426840446a0e0402c9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 18:25:29 +0100
Subject: [PATCH 12/23] Move logic into matcher

---
 .../UseDesignatedInitializersCheck.cpp        | 21 +++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index 0594f3dfb10306..28567e6609fdd3 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -12,6 +12,7 @@
 #include "clang/AST/Stmt.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/ASTMatchers/ASTMatchersMacros.h"
 #include <algorithm>
 #include <iterator>
 #include <vector>
@@ -40,24 +41,32 @@ UseDesignatedInitializersCheck::UseDesignatedInitializersCheck(
           Options.get(IgnoreSingleElementAggregatesName,
                       IgnoreSingleElementAggregatesDefault)) {}
 
+AST_MATCHER(CXXRecordDecl, isAggregate) { return Node.isAggregate(); }
+
+AST_MATCHER(InitListExpr, isFullyDesignated) {
+  return getUndesignatedComponents(&Node).empty();
+}
+
+AST_MATCHER(InitListExpr, hasSingleElement) { return Node.getNumInits() == 1; }
 void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
-      initListExpr(hasType(recordDecl().bind("type"))).bind("init"), this);
+      initListExpr(hasType(cxxRecordDecl(isAggregate()).bind("type")),
+                   unless(IgnoreSingleElementAggregates ? hasSingleElement()
+                                                        : unless(anything())),
+                   unless(isFullyDesignated()))
+          .bind("init"),
+      this);
 }
 
 void UseDesignatedInitializersCheck::check(
     const MatchFinder::MatchResult &Result) {
   const auto *InitList = Result.Nodes.getNodeAs<InitListExpr>("init");
   const auto *Type = Result.Nodes.getNodeAs<CXXRecordDecl>("type");
-  if (!Type || !InitList || !Type->isAggregate())
-    return;
-  if (IgnoreSingleElementAggregates && InitList->getNumInits() == 1)
+  if (!Type || !InitList)
     return;
   if (const auto *SyntacticInitList = InitList->getSyntacticForm()) {
     const auto UndesignatedComponents =
         getUndesignatedComponents(SyntacticInitList);
-    if (UndesignatedComponents.empty())
-      return;
     if (UndesignatedComponents.size() == SyntacticInitList->getNumInits()) {
       diag(InitList->getLBraceLoc(), "use designated initializer list");
       return;

>From 8b99092c44140f83e6d0a801f31f7829871936f6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sat, 3 Feb 2024 18:40:45 +0100
Subject: [PATCH 13/23] Add template test

---
 .../modernize/use-designated-initializers.cpp         | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index d932e2d5e4863b..b6f0c3758163d6 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -46,3 +46,14 @@ struct S4 {
 
 S4 s41 {2.2};
 // CHECK-MESSAGES-SINGLE-ELEMENT: :[[@LINE-1]]:8: warning: use designated initializer list [modernize-use-designated-initializers]
+
+S4 s42 = {{}};
+// CHECK-MESSAGES-SINGLE-ELEMENT: :[[@LINE-1]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
+
+template<typename S> S template1() { return {10, 11}; }
+
+S2 s26 = template1<S2>();
+
+template<typename S> S template2() { return {}; }
+
+S2 s27 = template2<S2>();

>From 30bfeae261373d30458f24769d6e577edef345a5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 4 Feb 2024 11:50:12 +0100
Subject: [PATCH 14/23] Ignore types inheriting fields

---
 .../modernize/UseDesignatedInitializersCheck.cpp  | 15 +++++++++++----
 .../modernize/use-designated-initializers.cpp     |  4 ++++
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index 28567e6609fdd3..62f2b59241dd70 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -48,12 +48,19 @@ AST_MATCHER(InitListExpr, isFullyDesignated) {
 }
 
 AST_MATCHER(InitListExpr, hasSingleElement) { return Node.getNumInits() == 1; }
+
+AST_MATCHER_FUNCTION(::internal::Matcher<CXXRecordDecl>, hasBaseWithFields) {
+  return hasAnyBase(hasType(cxxRecordDecl(has(fieldDecl()))));
+}
+
 void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
-      initListExpr(hasType(cxxRecordDecl(isAggregate()).bind("type")),
-                   unless(IgnoreSingleElementAggregates ? hasSingleElement()
-                                                        : unless(anything())),
-                   unless(isFullyDesignated()))
+      initListExpr(
+          hasType(cxxRecordDecl(isAggregate(), unless(hasBaseWithFields()))
+                      .bind("type")),
+          unless(IgnoreSingleElementAggregates ? hasSingleElement()
+                                               : unless(anything())),
+          unless(isFullyDesignated()))
           .bind("init"),
       this);
 }
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index b6f0c3758163d6..0a4a098a7737be 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -57,3 +57,7 @@ S2 s26 = template1<S2>();
 template<typename S> S template2() { return {}; }
 
 S2 s27 = template2<S2>();
+
+struct S5: S2 { int x, y; };
+
+S5 s51 {1, 2, .x = 3, .y = 4};

>From 25e62bd11d66d5623479accd62cfddaa4bd1b976 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 4 Feb 2024 13:21:26 +0100
Subject: [PATCH 15/23] Ignore types with anonymous fields

---
 .../UseDesignatedInitializersCheck.cpp        | 12 ++++++++++-
 .../modernize/use-designated-initializers.cpp | 20 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index 62f2b59241dd70..84fb59e5a76397 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "UseDesignatedInitializersCheck.h"
+#include "clang/AST/APValue.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/Stmt.h"
@@ -53,10 +54,19 @@ AST_MATCHER_FUNCTION(::internal::Matcher<CXXRecordDecl>, hasBaseWithFields) {
   return hasAnyBase(hasType(cxxRecordDecl(has(fieldDecl()))));
 }
 
+AST_MATCHER(FieldDecl, isAnonymousDecl) {
+  if (const auto *Record =
+          Node.getType().getCanonicalType()->getAsRecordDecl()) {
+    return Record->isAnonymousStructOrUnion() || !Record->getIdentifier();
+  }
+  return false;
+}
+
 void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
       initListExpr(
-          hasType(cxxRecordDecl(isAggregate(), unless(hasBaseWithFields()))
+          hasType(cxxRecordDecl(isAggregate(), unless(hasBaseWithFields()),
+                                unless(has(fieldDecl(isAnonymousDecl()))))
                       .bind("type")),
           unless(IgnoreSingleElementAggregates ? hasSingleElement()
                                                : unless(anything())),
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index 0a4a098a7737be..46859435c588e4 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -61,3 +61,23 @@ S2 s27 = template2<S2>();
 struct S5: S2 { int x, y; };
 
 S5 s51 {1, 2, .x = 3, .y = 4};
+
+struct S6 {
+    int i;
+    struct { int j; } s;
+};
+
+S6 s61 {1, 2};
+
+struct S7 {
+    union {
+        int k;
+        double d;
+    } u;
+};
+
+S7 s71 {1};
+
+struct S8: S7 { int i; };
+
+S8 s81{1, 2};

>From 728d7763fa125ff54f28bfc6ab4df58aac459029 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 4 Feb 2024 13:29:52 +0100
Subject: [PATCH 16/23] Remove restriction to C++ 20

---
 .../clang-tidy/modernize/UseDesignatedInitializersCheck.h   | 3 ---
 .../checks/modernize/use-designated-initializers.rst        | 6 +++++-
 .../checkers/modernize/use-designated-initializers.cpp      | 4 ++--
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
index de06250b0d3d06..54dca26c6d735a 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
@@ -28,9 +28,6 @@ class UseDesignatedInitializersCheck : public ClangTidyCheck {
   std::optional<TraversalKind> getCheckTraversalKind() const override {
     return TK_IgnoreUnlessSpelledInSource;
   }
-  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
-    return LangOpts.CPlusPlus20;
-  }
   void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
 
 private:
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
index 0f313eec2b1990..105daf2c50ef2e 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
@@ -12,7 +12,11 @@ arise when changing the order of fields.
 
 C++ 20 supports the designated initializer syntax for aggregate types.
 By applying it, we can always be sure that aggregates are constructed correctly,
-because the every variable being initialized is referenced by name.
+because every variable being initialized is referenced by name.
+
+Even when compiling in a language version older than C++ 20, depending on you compiler,
+designated initializers are potentially supported. Therefore, the check is not restricted
+to C++ 20 and older.
 
 Example:
 
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index 46859435c588e4..9780bf7f0080d0 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -1,5 +1,5 @@
-// RUN: %check_clang_tidy -std=c++20 %s modernize-use-designated-initializers %t
-// RUN: %check_clang_tidy -check-suffixes=,SINGLE-ELEMENT -std=c++20 %s modernize-use-designated-initializers %t \
+// RUN: %check_clang_tidy -std=c++17 %s modernize-use-designated-initializers %t
+// RUN: %check_clang_tidy -check-suffixes=,SINGLE-ELEMENT -std=c++17 %s modernize-use-designated-initializers %t \
 // RUN:     -- -config="{CheckOptions: [{key: modernize-use-designated-initializers.IgnoreSingleElementAggregates, value: false}]}" \
 // RUN:     --
 

>From d5fa47a104bb9c01691137691754c73765922183 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 4 Feb 2024 14:05:31 +0100
Subject: [PATCH 17/23] Add option to restrict check to POD types

---
 .../UseDesignatedInitializersCheck.cpp         | 18 +++++++++++++-----
 .../modernize/UseDesignatedInitializersCheck.h |  1 +
 .../modernize/use-designated-initializers.rst  |  6 ++++++
 .../modernize/use-designated-initializers.cpp  | 18 ++++++++++++++++++
 4 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index 84fb59e5a76397..b1b44289111c7a 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -26,6 +26,9 @@ static const char *IgnoreSingleElementAggregatesName =
     "IgnoreSingleElementAggregates";
 static const bool IgnoreSingleElementAggregatesDefault = true;
 
+static const char *RestrictToPODTypesName = "RestrictToPODTypes";
+static const bool RestrictToPODTypesDefault = false;
+
 static std::vector<Stmt *>
 getUndesignatedComponents(const InitListExpr *SyntacticInitList) {
   std::vector<Stmt *> Result;
@@ -37,13 +40,16 @@ getUndesignatedComponents(const InitListExpr *SyntacticInitList) {
 
 UseDesignatedInitializersCheck::UseDesignatedInitializersCheck(
     StringRef Name, ClangTidyContext *Context)
-    : ClangTidyCheck(Name, Context),
-      IgnoreSingleElementAggregates(
-          Options.get(IgnoreSingleElementAggregatesName,
-                      IgnoreSingleElementAggregatesDefault)) {}
+    : ClangTidyCheck(Name, Context), IgnoreSingleElementAggregates(Options.get(
+                                         IgnoreSingleElementAggregatesName,
+                                         IgnoreSingleElementAggregatesDefault)),
+      RestrictToPODTypes(
+          Options.get(RestrictToPODTypesName, RestrictToPODTypesDefault)) {}
 
 AST_MATCHER(CXXRecordDecl, isAggregate) { return Node.isAggregate(); }
 
+AST_MATCHER(CXXRecordDecl, isPOD) { return Node.isPOD(); }
+
 AST_MATCHER(InitListExpr, isFullyDesignated) {
   return getUndesignatedComponents(&Node).empty();
 }
@@ -65,7 +71,8 @@ AST_MATCHER(FieldDecl, isAnonymousDecl) {
 void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(
       initListExpr(
-          hasType(cxxRecordDecl(isAggregate(), unless(hasBaseWithFields()),
+          hasType(cxxRecordDecl(RestrictToPODTypes ? isPOD() : isAggregate(),
+                                unless(hasBaseWithFields()),
                                 unless(has(fieldDecl(isAnonymousDecl()))))
                       .bind("type")),
           unless(IgnoreSingleElementAggregates ? hasSingleElement()
@@ -98,6 +105,7 @@ void UseDesignatedInitializersCheck::storeOptions(
     ClangTidyOptions::OptionMap &Opts) {
   Options.store(Opts, IgnoreSingleElementAggregatesName,
                 IgnoreSingleElementAggregates);
+  Options.store(Opts, RestrictToPODTypesName, RestrictToPODTypes);
 }
 
 } // namespace clang::tidy::modernize
diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
index 54dca26c6d735a..9aa216d30ef6b6 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.h
@@ -32,6 +32,7 @@ class UseDesignatedInitializersCheck : public ClangTidyCheck {
 
 private:
   bool IgnoreSingleElementAggregates;
+  bool RestrictToPODTypes;
 };
 
 } // namespace clang::tidy::modernize
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
index 105daf2c50ef2e..a306e784c33562 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
@@ -46,3 +46,9 @@ Options
 
     The value `false` specifies that even initializers for aggregate types
     with only a single element should be checked. The default value is `true`.
+
+.. option:: RestrictToPODTypes
+
+    The value `true` specifies that only Plain Old Data (POD) types shall be
+    checked. This makes the check applicable to even older C++ standards.
+    The default value is `false`.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index 9780bf7f0080d0..e0b722ec091671 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -2,6 +2,9 @@
 // RUN: %check_clang_tidy -check-suffixes=,SINGLE-ELEMENT -std=c++17 %s modernize-use-designated-initializers %t \
 // RUN:     -- -config="{CheckOptions: [{key: modernize-use-designated-initializers.IgnoreSingleElementAggregates, value: false}]}" \
 // RUN:     --
+// RUN: %check_clang_tidy -check-suffixes=POD -std=c++17 %s modernize-use-designated-initializers %t \
+// RUN:     -- -config="{CheckOptions: [{key: modernize-use-designated-initializers.RestrictToPODTypes, value: true}]}" \
+// RUN:     --
 
 struct S1 {};
 
@@ -16,14 +19,17 @@ S2 s21{.i=1, .j =2};
 
 S2 s22 = {1, 2};
 // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-MESSAGES-POD: :[[@LINE-2]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
 
 S2 s23{1};
 // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-MESSAGES-POD: :[[@LINE-2]]:7: warning: use designated initializer list [modernize-use-designated-initializers]
 
 S2 s24{.i = 1};
 
 S2 s25 = {.i=1, 2};
 // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-MESSAGES-POD: :[[@LINE-2]]:17: warning: use designated init expression [modernize-use-designated-initializers]
 
 class S3 {
   public:
@@ -34,10 +40,14 @@ class S3 {
 S3 s31 = {.s2 = 1, 2, 3.1};
 // CHECK-MESSAGES: :[[@LINE-1]]:20: warning: use designated init expression [modernize-use-designated-initializers]
 // CHECK-MESSAGES: :[[@LINE-2]]:23: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-MESSAGES-POD: :[[@LINE-3]]:20: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-MESSAGES-POD: :[[@LINE-4]]:23: warning: use designated init expression [modernize-use-designated-initializers]
 
 S3 s32 = {{.i = 1, 2}};
 // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
 // CHECK-MESSAGES: :[[@LINE-2]]:20: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-MESSAGES-POD: :[[@LINE-3]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-MESSAGES-POD: :[[@LINE-4]]:20: warning: use designated init expression [modernize-use-designated-initializers]
 
 struct S4 {
     double d;
@@ -81,3 +91,11 @@ S7 s71 {1};
 struct S8: S7 { int i; };
 
 S8 s81{1, 2};
+
+struct S9 {
+    int i, j;
+    S9 &operator=(S9);
+};
+
+S9 s91{1, 2};
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use designated initializer list [modernize-use-designated-initializers]

>From 189e5bb867f60d6eff80fa73b8276098fb27ec3d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 11 Feb 2024 11:52:29 +0100
Subject: [PATCH 18/23] Add fix-its

---
 .../modernize/UseDesignatedInitializersCheck.cpp  | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index b1b44289111c7a..081286081b4cbf 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -95,8 +95,19 @@ void UseDesignatedInitializersCheck::check(
       diag(InitList->getLBraceLoc(), "use designated initializer list");
       return;
     }
-    for (const auto *InitExpr : UndesignatedComponents) {
-      diag(InitExpr->getBeginLoc(), "use designated init expression");
+    const auto FieldIterator = Type->fields().begin();
+    for (const auto *InitExpr : *SyntacticInitList) {
+      const auto Field = std::next(FieldIterator);
+      if (std::find(UndesignatedComponents.begin(),
+                    UndesignatedComponents.end(),
+                    InitExpr) == UndesignatedComponents.end())
+        continue;
+      if (const auto *FieldID = Field->getIdentifier()) {
+        const auto FieldName = FieldID->getName();
+        diag(InitExpr->getBeginLoc(), "use designated init expression")
+            << FixItHint::CreateInsertion(InitExpr->getBeginLoc(),
+                                          "." + FieldName.str() + "=");
+      }
     }
   }
 }

>From 9c53c3cbada877aec866cb4292f426fa1700eafb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 11 Feb 2024 14:36:19 +0100
Subject: [PATCH 19/23] Move computation of designated initializers to common
 place

---
 clang-tools-extra/clangd/InlayHints.cpp       | 167 +--------------
 .../clang/Tooling/DesignatedInitializers.h    |  43 ++++
 clang/lib/Tooling/CMakeLists.txt              |   1 +
 clang/lib/Tooling/DesignatedInitializers.cpp  | 195 ++++++++++++++++++
 4 files changed, 241 insertions(+), 165 deletions(-)
 create mode 100644 clang/include/clang/Tooling/DesignatedInitializers.h
 create mode 100644 clang/lib/Tooling/DesignatedInitializers.cpp

diff --git a/clang-tools-extra/clangd/InlayHints.cpp b/clang-tools-extra/clangd/InlayHints.cpp
index 671a9c30ffa959..09f5dba5697b10 100644
--- a/clang-tools-extra/clangd/InlayHints.cpp
+++ b/clang-tools-extra/clangd/InlayHints.cpp
@@ -23,8 +23,8 @@
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/OperatorKinds.h"
 #include "clang/Basic/SourceManager.h"
+#include "clang/Tooling/DesignatedInitializers.h"
 #include "llvm/ADT/DenseSet.h"
-#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
@@ -42,169 +42,6 @@ namespace {
 // For now, inlay hints are always anchored at the left or right of their range.
 enum class HintSide { Left, Right };
 
-// Helper class to iterate over the designator names of an aggregate type.
-//
-// For an array type, yields [0], [1], [2]...
-// For aggregate classes, yields null for each base, then .field1, .field2, ...
-class AggregateDesignatorNames {
-public:
-  AggregateDesignatorNames(QualType T) {
-    if (!T.isNull()) {
-      T = T.getCanonicalType();
-      if (T->isArrayType()) {
-        IsArray = true;
-        Valid = true;
-        return;
-      }
-      if (const RecordDecl *RD = T->getAsRecordDecl()) {
-        Valid = true;
-        FieldsIt = RD->field_begin();
-        FieldsEnd = RD->field_end();
-        if (const auto *CRD = llvm::dyn_cast<CXXRecordDecl>(RD)) {
-          BasesIt = CRD->bases_begin();
-          BasesEnd = CRD->bases_end();
-          Valid = CRD->isAggregate();
-        }
-        OneField = Valid && BasesIt == BasesEnd && FieldsIt != FieldsEnd &&
-                   std::next(FieldsIt) == FieldsEnd;
-      }
-    }
-  }
-  // Returns false if the type was not an aggregate.
-  operator bool() { return Valid; }
-  // Advance to the next element in the aggregate.
-  void next() {
-    if (IsArray)
-      ++Index;
-    else if (BasesIt != BasesEnd)
-      ++BasesIt;
-    else if (FieldsIt != FieldsEnd)
-      ++FieldsIt;
-  }
-  // Print the designator to Out.
-  // Returns false if we could not produce a designator for this element.
-  bool append(std::string &Out, bool ForSubobject) {
-    if (IsArray) {
-      Out.push_back('[');
-      Out.append(std::to_string(Index));
-      Out.push_back(']');
-      return true;
-    }
-    if (BasesIt != BasesEnd)
-      return false; // Bases can't be designated. Should we make one up?
-    if (FieldsIt != FieldsEnd) {
-      llvm::StringRef FieldName;
-      if (const IdentifierInfo *II = FieldsIt->getIdentifier())
-        FieldName = II->getName();
-
-      // For certain objects, their subobjects may be named directly.
-      if (ForSubobject &&
-          (FieldsIt->isAnonymousStructOrUnion() ||
-           // std::array<int,3> x = {1,2,3}. Designators not strictly valid!
-           (OneField && isReservedName(FieldName))))
-        return true;
-
-      if (!FieldName.empty() && !isReservedName(FieldName)) {
-        Out.push_back('.');
-        Out.append(FieldName.begin(), FieldName.end());
-        return true;
-      }
-      return false;
-    }
-    return false;
-  }
-
-private:
-  bool Valid = false;
-  bool IsArray = false;
-  bool OneField = false; // e.g. std::array { T __elements[N]; }
-  unsigned Index = 0;
-  CXXRecordDecl::base_class_const_iterator BasesIt;
-  CXXRecordDecl::base_class_const_iterator BasesEnd;
-  RecordDecl::field_iterator FieldsIt;
-  RecordDecl::field_iterator FieldsEnd;
-};
-
-// Collect designator labels describing the elements of an init list.
-//
-// This function contributes the designators of some (sub)object, which is
-// represented by the semantic InitListExpr Sem.
-// This includes any nested subobjects, but *only* if they are part of the same
-// original syntactic init list (due to brace elision).
-// In other words, it may descend into subobjects but not written init-lists.
-//
-// For example: struct Outer { Inner a,b; }; struct Inner { int x, y; }
-//              Outer o{{1, 2}, 3};
-// This function will be called with Sem = { {1, 2}, {3, ImplicitValue} }
-// It should generate designators '.a:' and '.b.x:'.
-// '.a:' is produced directly without recursing into the written sublist.
-// (The written sublist will have a separate collectDesignators() call later).
-// Recursion with Prefix='.b' and Sem = {3, ImplicitValue} produces '.b.x:'.
-void collectDesignators(const InitListExpr *Sem,
-                        llvm::DenseMap<SourceLocation, std::string> &Out,
-                        const llvm::DenseSet<SourceLocation> &NestedBraces,
-                        std::string &Prefix) {
-  if (!Sem || Sem->isTransparent())
-    return;
-  assert(Sem->isSemanticForm());
-
-  // The elements of the semantic form all correspond to direct subobjects of
-  // the aggregate type. `Fields` iterates over these subobject names.
-  AggregateDesignatorNames Fields(Sem->getType());
-  if (!Fields)
-    return;
-  for (const Expr *Init : Sem->inits()) {
-    auto Next = llvm::make_scope_exit([&, Size(Prefix.size())] {
-      Fields.next();       // Always advance to the next subobject name.
-      Prefix.resize(Size); // Erase any designator we appended.
-    });
-    // Skip for a broken initializer or if it is a "hole" in a subobject that
-    // was not explicitly initialized.
-    if (!Init || llvm::isa<ImplicitValueInitExpr>(Init))
-      continue;
-
-    const auto *BraceElidedSubobject = llvm::dyn_cast<InitListExpr>(Init);
-    if (BraceElidedSubobject &&
-        NestedBraces.contains(BraceElidedSubobject->getLBraceLoc()))
-      BraceElidedSubobject = nullptr; // there were braces!
-
-    if (!Fields.append(Prefix, BraceElidedSubobject != nullptr))
-      continue; // no designator available for this subobject
-    if (BraceElidedSubobject) {
-      // If the braces were elided, this aggregate subobject is initialized
-      // inline in the same syntactic list.
-      // Descend into the semantic list describing the subobject.
-      // (NestedBraces are still correct, they're from the same syntactic list).
-      collectDesignators(BraceElidedSubobject, Out, NestedBraces, Prefix);
-      continue;
-    }
-    Out.try_emplace(Init->getBeginLoc(), Prefix);
-  }
-}
-
-// Get designators describing the elements of a (syntactic) init list.
-// This does not produce designators for any explicitly-written nested lists.
-llvm::DenseMap<SourceLocation, std::string>
-getDesignators(const InitListExpr *Syn) {
-  assert(Syn->isSyntacticForm());
-
-  // collectDesignators needs to know which InitListExprs in the semantic tree
-  // were actually written, but InitListExpr::isExplicit() lies.
-  // Instead, record where braces of sub-init-lists occur in the syntactic form.
-  llvm::DenseSet<SourceLocation> NestedBraces;
-  for (const Expr *Init : Syn->inits())
-    if (auto *Nested = llvm::dyn_cast<InitListExpr>(Init))
-      NestedBraces.insert(Nested->getLBraceLoc());
-
-  // Traverse the semantic form to find the designators.
-  // We use their SourceLocation to correlate with the syntactic form later.
-  llvm::DenseMap<SourceLocation, std::string> Designators;
-  std::string EmptyPrefix;
-  collectDesignators(Syn->isSemanticForm() ? Syn : Syn->getSemanticForm(),
-                     Designators, NestedBraces, EmptyPrefix);
-  return Designators;
-}
-
 void stripLeadingUnderscores(StringRef &Name) { Name = Name.ltrim('_'); }
 
 // getDeclForType() returns the decl responsible for Type's spelling.
@@ -854,7 +691,7 @@ class InlayHintVisitor : public RecursiveASTVisitor<InlayHintVisitor> {
     if (Syn->isIdiomaticZeroInitializer(AST.getLangOpts()))
       return true;
     llvm::DenseMap<SourceLocation, std::string> Designators =
-        getDesignators(Syn);
+        clang::tooling::getDesignators(Syn);
     for (const Expr *Init : Syn->inits()) {
       if (llvm::isa<DesignatedInitExpr>(Init))
         continue;
diff --git a/clang/include/clang/Tooling/DesignatedInitializers.h b/clang/include/clang/Tooling/DesignatedInitializers.h
new file mode 100644
index 00000000000000..e978dfe639f0c0
--- /dev/null
+++ b/clang/include/clang/Tooling/DesignatedInitializers.h
@@ -0,0 +1,43 @@
+//===--- DesignatedInitializers.h -------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file provides utilities for designated initializers.
+///
+//===----------------------------------------------------------------------===//
+
+#include "clang/AST/Expr.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/DenseMap.h"
+
+namespace clang::tooling {
+
+/// Get designators describing the elements of a (syntactic) init list.
+///
+/// Given for example the type
+///
+///     struct S { int i, j; };
+///
+/// and the definition
+///
+///     S s{1, 2};
+///
+/// calling `getDesignators` for the initializer list expression `{1, 2}`
+/// would produce the map `{loc(1): ".i", loc(2): ".j"}`.
+///
+/// It does not produce designators for any explicitly-written nested lists,
+/// e.g. `{1, .j=2}` would only return `{loc(1): ".i"}`.
+///
+/// It also considers structs with fields of record types like
+/// `struct T { S s; };`. In this case, there would be designators of the
+/// form
+/// `.s.i` and `.s.j` in the returned map.
+llvm::DenseMap<clang::SourceLocation, std::string>
+getDesignators(const clang::InitListExpr *Syn);
+
+} // namespace clang::tooling
diff --git a/clang/lib/Tooling/CMakeLists.txt b/clang/lib/Tooling/CMakeLists.txt
index aff39e4de13c0b..8b0853d9150bb3 100644
--- a/clang/lib/Tooling/CMakeLists.txt
+++ b/clang/lib/Tooling/CMakeLists.txt
@@ -106,6 +106,7 @@ add_clang_library(clangTooling
   ArgumentsAdjusters.cpp
   CommonOptionsParser.cpp
   CompilationDatabase.cpp
+  DesignatedInitializers.cpp
   Execution.cpp
   ExpandResponseFilesCompilationDatabase.cpp
   FileMatchTrie.cpp
diff --git a/clang/lib/Tooling/DesignatedInitializers.cpp b/clang/lib/Tooling/DesignatedInitializers.cpp
new file mode 100644
index 00000000000000..8671625bd19ea3
--- /dev/null
+++ b/clang/lib/Tooling/DesignatedInitializers.cpp
@@ -0,0 +1,195 @@
+//===--- DesignatedInitializers.cpp ---------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file provides utilities for designated initializers.
+///
+//===----------------------------------------------------------------------===//
+
+#include "clang/Tooling/DesignatedInitializers.h"
+#include "clang/AST/DeclCXX.h"
+#include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/ScopeExit.h"
+
+namespace clang::tooling {
+
+namespace {
+
+/// Returns true if Name is reserved, like _Foo or __Vector_base.
+static inline bool isReservedName(llvm::StringRef Name) {
+  // This doesn't catch all cases, but the most common.
+  return Name.size() >= 2 && Name[0] == '_' &&
+         (isUppercase(Name[1]) || Name[1] == '_');
+}
+
+// Helper class to iterate over the designator names of an aggregate type.
+//
+// For an array type, yields [0], [1], [2]...
+// For aggregate classes, yields null for each base, then .field1, .field2,
+// ...
+class AggregateDesignatorNames {
+public:
+  AggregateDesignatorNames(QualType T) {
+    if (!T.isNull()) {
+      T = T.getCanonicalType();
+      if (T->isArrayType()) {
+        IsArray = true;
+        Valid = true;
+        return;
+      }
+      if (const RecordDecl *RD = T->getAsRecordDecl()) {
+        Valid = true;
+        FieldsIt = RD->field_begin();
+        FieldsEnd = RD->field_end();
+        if (const auto *CRD = llvm::dyn_cast<CXXRecordDecl>(RD)) {
+          BasesIt = CRD->bases_begin();
+          BasesEnd = CRD->bases_end();
+          Valid = CRD->isAggregate();
+        }
+        OneField = Valid && BasesIt == BasesEnd && FieldsIt != FieldsEnd &&
+                   std::next(FieldsIt) == FieldsEnd;
+      }
+    }
+  }
+  // Returns false if the type was not an aggregate.
+  operator bool() { return Valid; }
+  // Advance to the next element in the aggregate.
+  void next() {
+    if (IsArray)
+      ++Index;
+    else if (BasesIt != BasesEnd)
+      ++BasesIt;
+    else if (FieldsIt != FieldsEnd)
+      ++FieldsIt;
+  }
+  // Print the designator to Out.
+  // Returns false if we could not produce a designator for this element.
+  bool append(std::string &Out, bool ForSubobject) {
+    if (IsArray) {
+      Out.push_back('[');
+      Out.append(std::to_string(Index));
+      Out.push_back(']');
+      return true;
+    }
+    if (BasesIt != BasesEnd)
+      return false; // Bases can't be designated. Should we make one up?
+    if (FieldsIt != FieldsEnd) {
+      llvm::StringRef FieldName;
+      if (const IdentifierInfo *II = FieldsIt->getIdentifier())
+        FieldName = II->getName();
+
+      // For certain objects, their subobjects may be named directly.
+      if (ForSubobject &&
+          (FieldsIt->isAnonymousStructOrUnion() ||
+           // std::array<int,3> x = {1,2,3}. Designators not strictly valid!
+           (OneField && isReservedName(FieldName))))
+        return true;
+
+      if (!FieldName.empty() && !isReservedName(FieldName)) {
+        Out.push_back('.');
+        Out.append(FieldName.begin(), FieldName.end());
+        return true;
+      }
+      return false;
+    }
+    return false;
+  }
+
+private:
+  bool Valid = false;
+  bool IsArray = false;
+  bool OneField = false; // e.g. std::array { T __elements[N]; }
+  unsigned Index = 0;
+  CXXRecordDecl::base_class_const_iterator BasesIt;
+  CXXRecordDecl::base_class_const_iterator BasesEnd;
+  RecordDecl::field_iterator FieldsIt;
+  RecordDecl::field_iterator FieldsEnd;
+};
+
+// Collect designator labels describing the elements of an init list.
+//
+// This function contributes the designators of some (sub)object, which is
+// represented by the semantic InitListExpr Sem.
+// This includes any nested subobjects, but *only* if they are part of the
+// same original syntactic init list (due to brace elision). In other words,
+// it may descend into subobjects but not written init-lists.
+//
+// For example: struct Outer { Inner a,b; }; struct Inner { int x, y; }
+//              Outer o{{1, 2}, 3};
+// This function will be called with Sem = { {1, 2}, {3, ImplicitValue} }
+// It should generate designators '.a:' and '.b.x:'.
+// '.a:' is produced directly without recursing into the written sublist.
+// (The written sublist will have a separate collectDesignators() call later).
+// Recursion with Prefix='.b' and Sem = {3, ImplicitValue} produces '.b.x:'.
+void collectDesignators(const InitListExpr *Sem,
+                        llvm::DenseMap<SourceLocation, std::string> &Out,
+                        const llvm::DenseSet<SourceLocation> &NestedBraces,
+                        std::string &Prefix) {
+  if (!Sem || Sem->isTransparent())
+    return;
+  assert(Sem->isSemanticForm());
+
+  // The elements of the semantic form all correspond to direct subobjects of
+  // the aggregate type. `Fields` iterates over these subobject names.
+  AggregateDesignatorNames Fields(Sem->getType());
+  if (!Fields)
+    return;
+  for (const Expr *Init : Sem->inits()) {
+    auto Next = llvm::make_scope_exit([&, Size(Prefix.size())] {
+      Fields.next();       // Always advance to the next subobject name.
+      Prefix.resize(Size); // Erase any designator we appended.
+    });
+    // Skip for a broken initializer or if it is a "hole" in a subobject that
+    // was not explicitly initialized.
+    if (!Init || llvm::isa<ImplicitValueInitExpr>(Init))
+      continue;
+
+    const auto *BraceElidedSubobject = llvm::dyn_cast<InitListExpr>(Init);
+    if (BraceElidedSubobject &&
+        NestedBraces.contains(BraceElidedSubobject->getLBraceLoc()))
+      BraceElidedSubobject = nullptr; // there were braces!
+
+    if (!Fields.append(Prefix, BraceElidedSubobject != nullptr))
+      continue; // no designator available for this subobject
+    if (BraceElidedSubobject) {
+      // If the braces were elided, this aggregate subobject is initialized
+      // inline in the same syntactic list.
+      // Descend into the semantic list describing the subobject.
+      // (NestedBraces are still correct, they're from the same syntactic
+      // list).
+      collectDesignators(BraceElidedSubobject, Out, NestedBraces, Prefix);
+      continue;
+    }
+    Out.try_emplace(Init->getBeginLoc(), Prefix);
+  }
+}
+
+} // namespace
+
+llvm::DenseMap<SourceLocation, std::string>
+getDesignators(const InitListExpr *Syn) {
+  assert(Syn->isSyntacticForm());
+
+  // collectDesignators needs to know which InitListExprs in the semantic tree
+  // were actually written, but InitListExpr::isExplicit() lies.
+  // Instead, record where braces of sub-init-lists occur in the syntactic form.
+  llvm::DenseSet<SourceLocation> NestedBraces;
+  for (const Expr *Init : Syn->inits())
+    if (auto *Nested = llvm::dyn_cast<InitListExpr>(Init))
+      NestedBraces.insert(Nested->getLBraceLoc());
+
+  // Traverse the semantic form to find the designators.
+  // We use their SourceLocation to correlate with the syntactic form later.
+  llvm::DenseMap<SourceLocation, std::string> Designators;
+  std::string EmptyPrefix;
+  collectDesignators(Syn->isSemanticForm() ? Syn : Syn->getSemanticForm(),
+                     Designators, NestedBraces, EmptyPrefix);
+  return Designators;
+}
+
+} // namespace clang::tooling

>From 42b9448bc25220dd4c8b544c02648fea10ddd87f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 11 Feb 2024 14:37:45 +0100
Subject: [PATCH 20/23] Fully implement fix-its reusing designated initializer
 computation from Clangd

---
 .../UseDesignatedInitializersCheck.cpp        | 67 ++++++++++---------
 .../modernize/use-designated-initializers.cpp |  8 +++
 2 files changed, 44 insertions(+), 31 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
index 081286081b4cbf..9e90df711df66a 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp
@@ -14,9 +14,9 @@
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/ASTMatchers/ASTMatchersMacros.h"
-#include <algorithm>
-#include <iterator>
-#include <vector>
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Lex/Lexer.h"
+#include "clang/Tooling/DesignatedInitializers.h"
 
 using namespace clang::ast_matchers;
 
@@ -29,15 +29,6 @@ static const bool IgnoreSingleElementAggregatesDefault = true;
 static const char *RestrictToPODTypesName = "RestrictToPODTypes";
 static const bool RestrictToPODTypesDefault = false;
 
-static std::vector<Stmt *>
-getUndesignatedComponents(const InitListExpr *SyntacticInitList) {
-  std::vector<Stmt *> Result;
-  std::copy_if(SyntacticInitList->begin(), SyntacticInitList->end(),
-               std::back_inserter(Result),
-               [](auto S) { return !isa<DesignatedInitExpr>(S); });
-  return Result;
-}
-
 UseDesignatedInitializersCheck::UseDesignatedInitializersCheck(
     StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context), IgnoreSingleElementAggregates(Options.get(
@@ -51,7 +42,9 @@ AST_MATCHER(CXXRecordDecl, isAggregate) { return Node.isAggregate(); }
 AST_MATCHER(CXXRecordDecl, isPOD) { return Node.isPOD(); }
 
 AST_MATCHER(InitListExpr, isFullyDesignated) {
-  return getUndesignatedComponents(&Node).empty();
+  return std::all_of(Node.begin(), Node.end(), [](auto *InitExpr) {
+    return isa<DesignatedInitExpr>(InitExpr);
+  });
 }
 
 AST_MATCHER(InitListExpr, hasSingleElement) { return Node.getNumInits() == 1; }
@@ -82,6 +75,12 @@ void UseDesignatedInitializersCheck::registerMatchers(MatchFinder *Finder) {
       this);
 }
 
+static bool isFullyUndesignated(const InitListExpr *SyntacticInitList) {
+  return std::all_of(
+      SyntacticInitList->begin(), SyntacticInitList->end(),
+      [](auto *InitExpr) { return !isa<DesignatedInitExpr>(InitExpr); });
+}
+
 void UseDesignatedInitializersCheck::check(
     const MatchFinder::MatchResult &Result) {
   const auto *InitList = Result.Nodes.getNodeAs<InitListExpr>("init");
@@ -89,24 +88,30 @@ void UseDesignatedInitializersCheck::check(
   if (!Type || !InitList)
     return;
   if (const auto *SyntacticInitList = InitList->getSyntacticForm()) {
-    const auto UndesignatedComponents =
-        getUndesignatedComponents(SyntacticInitList);
-    if (UndesignatedComponents.size() == SyntacticInitList->getNumInits()) {
-      diag(InitList->getLBraceLoc(), "use designated initializer list");
-      return;
-    }
-    const auto FieldIterator = Type->fields().begin();
-    for (const auto *InitExpr : *SyntacticInitList) {
-      const auto Field = std::next(FieldIterator);
-      if (std::find(UndesignatedComponents.begin(),
-                    UndesignatedComponents.end(),
-                    InitExpr) == UndesignatedComponents.end())
-        continue;
-      if (const auto *FieldID = Field->getIdentifier()) {
-        const auto FieldName = FieldID->getName();
-        diag(InitExpr->getBeginLoc(), "use designated init expression")
-            << FixItHint::CreateInsertion(InitExpr->getBeginLoc(),
-                                          "." + FieldName.str() + "=");
+    const llvm::DenseMap<clang::SourceLocation, std::string> Designators =
+        clang::tooling::getDesignators(SyntacticInitList);
+    if (isFullyUndesignated(SyntacticInitList)) {
+      std::string NewList = "{";
+      for (const Stmt *InitExpr : *SyntacticInitList) {
+        if (InitExpr != *SyntacticInitList->begin())
+          NewList += ", ";
+        NewList += Designators.at(InitExpr->getBeginLoc());
+        NewList += "=";
+        NewList += Lexer::getSourceText(
+            CharSourceRange::getTokenRange(InitExpr->getSourceRange()),
+            *Result.SourceManager, getLangOpts());
+      }
+      NewList += "}";
+      diag(InitList->getLBraceLoc(), "use designated initializer list")
+          << FixItHint::CreateReplacement(InitList->getSourceRange(), NewList);
+    } else {
+      for (const auto *InitExpr : *SyntacticInitList) {
+        if (!isa<DesignatedInitExpr>(InitExpr)) {
+          diag(InitExpr->getBeginLoc(), "use designated init expression")
+              << FixItHint::CreateInsertion(
+                     InitExpr->getBeginLoc(),
+                     Designators.at(InitExpr->getBeginLoc()) + "=");
+        }
       }
     }
   }
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index e0b722ec091671..7d8c2ee63b4aef 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -20,16 +20,19 @@ S2 s21{.i=1, .j =2};
 S2 s22 = {1, 2};
 // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
 // CHECK-MESSAGES-POD: :[[@LINE-2]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-FIXES: S2 s22 = {.i=1, .j=2};
 
 S2 s23{1};
 // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use designated initializer list [modernize-use-designated-initializers]
 // CHECK-MESSAGES-POD: :[[@LINE-2]]:7: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-FIXES: S2 s23{.i=1};
 
 S2 s24{.i = 1};
 
 S2 s25 = {.i=1, 2};
 // CHECK-MESSAGES: :[[@LINE-1]]:17: warning: use designated init expression [modernize-use-designated-initializers]
 // CHECK-MESSAGES-POD: :[[@LINE-2]]:17: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-FIXES: S2 s25 = {.i=1, .j=2};
 
 class S3 {
   public:
@@ -42,12 +45,14 @@ S3 s31 = {.s2 = 1, 2, 3.1};
 // CHECK-MESSAGES: :[[@LINE-2]]:23: warning: use designated init expression [modernize-use-designated-initializers]
 // CHECK-MESSAGES-POD: :[[@LINE-3]]:20: warning: use designated init expression [modernize-use-designated-initializers]
 // CHECK-MESSAGES-POD: :[[@LINE-4]]:23: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-FIXES: S3 s31 = {.s2 = 1, .s2.j=2, .d=3.1};
 
 S3 s32 = {{.i = 1, 2}};
 // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
 // CHECK-MESSAGES: :[[@LINE-2]]:20: warning: use designated init expression [modernize-use-designated-initializers]
 // CHECK-MESSAGES-POD: :[[@LINE-3]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
 // CHECK-MESSAGES-POD: :[[@LINE-4]]:20: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-FIXES: S3 s32 = {.s2={.i = 1, 2}};
 
 struct S4 {
     double d;
@@ -56,9 +61,11 @@ struct S4 {
 
 S4 s41 {2.2};
 // CHECK-MESSAGES-SINGLE-ELEMENT: :[[@LINE-1]]:8: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-FIXES-SINGLE-ELEMENT: S4 s41 {.d=2.2};
 
 S4 s42 = {{}};
 // CHECK-MESSAGES-SINGLE-ELEMENT: :[[@LINE-1]]:10: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-FIXES-SINGLE-ELEMENT: S4 s42 = {.d={}};
 
 template<typename S> S template1() { return {10, 11}; }
 
@@ -99,3 +106,4 @@ struct S9 {
 
 S9 s91{1, 2};
 // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use designated initializer list [modernize-use-designated-initializers]
+// CHECK-FIXES: S9 s91{.i=1, .j=2};

>From c2bb32a2740887b03b1927be997a55cd92ee98cb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 11 Feb 2024 14:46:21 +0100
Subject: [PATCH 21/23] Separate by empty line

---
 clang-tools-extra/docs/ReleaseNotes.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 3effe9f9d5b5f0..5d532e3cfb9a9c 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -105,6 +105,7 @@ New checks
 
   Replaces certain conditional statements with equivalent calls to
   ``std::min`` or ``std::max``.
+
 - New :doc:`modernize-use-designated-initializers
   <clang-tidy/checks/modernize/use-designated-initializers>` check.
 

>From 606a1b75928cd6f7cfbedf1dcfd3c09eb45e9adb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 11 Feb 2024 15:00:18 +0100
Subject: [PATCH 22/23] Add test for default-initialized fields

---
 .../checkers/modernize/use-designated-initializers.cpp      | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
index 7d8c2ee63b4aef..5eada72e225411 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-designated-initializers.cpp
@@ -107,3 +107,9 @@ struct S9 {
 S9 s91{1, 2};
 // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use designated initializer list [modernize-use-designated-initializers]
 // CHECK-FIXES: S9 s91{.i=1, .j=2};
+
+struct S10 { int i = 1, j = 2; };
+
+S10 s101 {1, .j=2};
+// CHECK-MESSAGES: :[[@LINE-1]]:11: warning: use designated init expression [modernize-use-designated-initializers]
+// CHECK-FIXES: S10 s101 {.i=1, .j=2};

>From c758b459ec9c06d2b32de40af72c8f61ae639fd3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Danny=20M=C3=B6sch?= <danny.moesch at icloud.com>
Date: Sun, 11 Feb 2024 17:13:37 +0100
Subject: [PATCH 23/23] Fix C++20 style

---
 .../checks/modernize/use-designated-initializers.rst        | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
index a306e784c33562..c17aea84b73941 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-designated-initializers.rst
@@ -10,13 +10,13 @@ With plain initializer lists, it is very easy to introduce bugs when adding
 new fields in the middle of a struct or class type. The same confusion might
 arise when changing the order of fields.
 
-C++ 20 supports the designated initializer syntax for aggregate types.
+C++20 supports the designated initializer syntax for aggregate types.
 By applying it, we can always be sure that aggregates are constructed correctly,
 because every variable being initialized is referenced by name.
 
-Even when compiling in a language version older than C++ 20, depending on you compiler,
+Even when compiling in a language version older than C++20, depending on you compiler,
 designated initializers are potentially supported. Therefore, the check is not restricted
-to C++ 20 and older.
+to C++20 and older.
 
 Example: