[clang] [clang][Sema] Fix crash on designated initializer of union member in subobject (PR #167171)

[Naveen Seth Hanig via cfe-commits]

https://github.com/naveen-seth updated https://github.com/llvm/llvm-project/pull/167171

>From afcda23927bb79dfd7446aa90bfc741d5254c2e2 Mon Sep 17 00:00:00 2001
From: naveen-seth <naveen.hanig at outlook.com>
Date: Sat, 8 Nov 2025 19:53:08 +0100
Subject: [PATCH 1/2] [clang][Sema] Fix crash on designated initializer of
 union member in subobject

Fixes #166327

Clang previously hit an assertion in C++ mode when a nested
initializer list was followed by a designated initializer that
referred to a union member of the same subobject.
This fixes the assertion.
---
 clang/lib/Sema/SemaInit.cpp                   |  5 +++-
 .../crash-union-designated-initializer.cpp    | 23 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/crash-union-designated-initializer.cpp

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index cc6ddf568d346..13fb22ebfe0ef 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -2402,8 +2402,11 @@ void InitListChecker::CheckStructUnionTypes(
         CheckEmptyInitializable(
             InitializedEntity::InitializeMember(*Field, &Entity),
             IList->getEndLoc());
-        if (StructuredList)
+        if (StructuredList) {
           StructuredList->setInitializedFieldInUnion(*Field);
+          StructuredList->resizeInits(SemaRef.Context,
+                                      StructuredList->getNumInits() + 1);
+        }
         break;
       }
     }
diff --git a/clang/test/SemaCXX/crash-union-designated-initializer.cpp b/clang/test/SemaCXX/crash-union-designated-initializer.cpp
new file mode 100644
index 0000000000000..7ba40311eac0b
--- /dev/null
+++ b/clang/test/SemaCXX/crash-union-designated-initializer.cpp
@@ -0,0 +1,23 @@
+// Ensures that Clang does not crash in C++ mode, when a nested initializer
+// is followed by a designated initializer for a union member of that same
+// subobject.
+// See issue #166327.
+
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+auto main(void) -> int {
+  struct Point {
+    float x;
+    float y;
+    union {
+      int idx;
+      char label;
+    } extra;
+  };
+
+  struct SavePoint {
+    struct Point p;
+  };
+
+  SavePoint save = {.p = {.x = 3.0, .y = 4.0}, .p.extra.label = 'p'}; // expected-warning {{nested designators are a C99 extension}}
+}

>From 3fbca9a0595292e9309747b29a27cace266c2b84 Mon Sep 17 00:00:00 2001
From: naveen-seth <naveen.hanig at outlook.com>
Date: Sat, 8 Nov 2025 21:02:47 +0100
Subject: [PATCH 2/2] Revert back to previous strategy to fix failing tests

---
 clang/lib/Sema/SemaInit.cpp | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 13fb22ebfe0ef..11cb9527525a9 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -2402,11 +2402,8 @@ void InitListChecker::CheckStructUnionTypes(
         CheckEmptyInitializable(
             InitializedEntity::InitializeMember(*Field, &Entity),
             IList->getEndLoc());
-        if (StructuredList) {
+        if (StructuredList)
           StructuredList->setInitializedFieldInUnion(*Field);
-          StructuredList->resizeInits(SemaRef.Context,
-                                      StructuredList->getNumInits() + 1);
-        }
         break;
       }
     }
@@ -3032,10 +3029,11 @@ InitListChecker::CheckDesignatedInitializer(const InitializedEntity &Entity,
       if (StructuredList) {
         FieldDecl *CurrentField = StructuredList->getInitializedFieldInUnion();
         if (CurrentField && !declaresSameEntity(CurrentField, *Field)) {
-          assert(StructuredList->getNumInits() == 1
-                 && "A union should never have more than one initializer!");
+          const auto NumInits = StructuredList->getNumInits();
+          assert(StructuredList->getNumInits() <= 1 &&
+                 "A union should never have more than one initializer!");
 
-          Expr *ExistingInit = StructuredList->getInit(0);
+          Expr *ExistingInit = NumInits ? StructuredList->getInit(0) : nullptr;
           if (ExistingInit) {
             // We're about to throw away an initializer, emit warning.
             diagnoseInitOverride(