[SemaCXX] Recognise initializer_list injected-class-name types as initializer_lists (#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

Author: MitalAshok

clang/docs/ReleaseNotes.rst

@@ -590,6 +590,8 @@ 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDeclCXX.cpp

@@ -12054,11 +12054,17 @@ 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;

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:

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);