[clang] [SemaCXX] Recognise initializer_list injected-class-name types as initializer_lists (PR #90210)

[Mital Ashok via cfe-commits]

https://github.com/MitalAshok created https://github.com/llvm/llvm-project/pull/90210

This allows the implicitly-generated deduction guide for the copy constructor to be recognised as an initializer-list constructor, allowing CTAD for std::initializer_list

>From ff65636895e8bceb5b4d6497a0e9ccda0c5fa22a Mon Sep 17 00:00:00 2001
From: Mital Ashok <mital at mitalashok.co.uk>
Date: Fri, 26 Apr 2024 13:44:17 +0100
Subject: [PATCH] [SemaCXX] Recognise initializer_list injected-class-name
 types as initializer_lists

This allows the implicitly-generated deduction guide for the copy constructor to be recognised as an initializer-list constructor, allowing CTAD for std::initializer_list
---
 clang/docs/ReleaseNotes.rst                      |  2 ++
 clang/lib/Sema/SemaDeclCXX.cpp                   | 16 +++++++++++-----
 clang/lib/Sema/SemaInit.cpp                      |  2 --
 .../cxx1z-class-template-argument-deduction.cpp  | 11 ++++++++---
 4 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 92563262cc6737..4065de5745aae6 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -581,6 +581,8 @@ Bug Fixes to C++ Support
 - Fixed a use-after-free bug in parsing of type constraints with default arguments that involve lambdas. (#GH67235)
 - Fixed bug in which the body of a consteval lambda within a template was not parsed as within an
   immediate function context.
+- Fix CTAD for ``std::initializer_list``. This allows ``std::initializer_list{1, 2, 3}`` to be deduced as
+  ``std::initializer_list<int>`` as intended.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index abdbc9d8830c03..8d98f593817bda 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -12052,11 +12052,17 @@ bool Sema::isStdInitializerList(QualType Ty, QualType *Element) {
 
     Template = Specialization->getSpecializedTemplate();
     Arguments = Specialization->getTemplateArgs().data();
-  } else if (const TemplateSpecializationType *TST =
-                 Ty->getAs<TemplateSpecializationType>()) {
-    Template = dyn_cast_or_null<ClassTemplateDecl>(
-        TST->getTemplateName().getAsTemplateDecl());
-    Arguments = TST->template_arguments().begin();
+  } else {
+    const TemplateSpecializationType *TST = nullptr;
+    if (auto *ICN = Ty->getAs<InjectedClassNameType>())
+      TST = ICN->getInjectedTST();
+    else
+      TST = Ty->getAs<TemplateSpecializationType>();
+    if (TST) {
+      Template = dyn_cast_or_null<ClassTemplateDecl>(
+          TST->getTemplateName().getAsTemplateDecl());
+      Arguments = TST->template_arguments().begin();
+    }
   }
   if (!Template)
     return false;
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 003a157990d307..842d165c60d94c 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -10790,8 +10790,6 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
   // FIXME: Perform "exact type" matching first, per CWG discussion?
   //        Or implement this via an implied 'T(T) -> T' deduction guide?
 
-  // FIXME: Do we need/want a std::initializer_list<T> special case?
-
   // Look up deduction guides, including those synthesized from constructors.
   //
   // C++1z [over.match.class.deduct]p1:
diff --git a/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp b/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
index 2f067ea53a5029..90404f115c75f7 100644
--- a/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
+++ b/clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
@@ -12,14 +12,19 @@ namespace std {
     size_t n;
     initializer_list();
   };
-  // FIXME: This should probably not be necessary.
-  template<typename T> initializer_list(initializer_list<T>) -> initializer_list<T>;
 }
 
 template<typename T> constexpr bool has_type(...) { return false; }
 template<typename T> constexpr bool has_type(T&) { return true; }
 
-std::initializer_list il = {1, 2, 3, 4, 5};
+std::initializer_list il1 = {1, 2, 3, 4, 5};
+auto il2 = std::initializer_list{1, 2, 3, 4};
+auto il3 = std::initializer_list{il1};
+auto il4 = std::initializer_list{il1, il1, il1};
+static_assert(has_type<std::initializer_list<int>>(il1));
+static_assert(has_type<std::initializer_list<int>>(il2));
+static_assert(has_type<std::initializer_list<int>>(il3));
+static_assert(has_type<std::initializer_list<std::initializer_list<int>>>(il4));
 
 template<typename T> struct vector {
   template<typename Iter> vector(Iter, Iter);