[clang] Fix crash on invalid std::initializer_list<T> template-id (#132284)

In `Sema::BuildStdInitializerList`, check that the synthesized
template-id `std::initializer_list<T>` is valid (which might not be the
case if the template has associated constraints or subsequent parameters
with default arguments) before forming the type.

Fixes #132256

Author: offsetof

clang/docs/ReleaseNotes.rst

@@ -371,6 +371,7 @@ Bug Fixes to C++ Support
 - Fixed a Clang regression in C++20 mode where unresolved dependent call expressions were created inside non-dependent contexts (#GH122892)
 - Clang now emits the ``-Wunused-variable`` warning when some structured bindings are unused
   and the ``[[maybe_unused]]`` attribute is not applied. (#GH125810)
+- Fixed a crash caused by invalid declarations of ``std::initializer_list``. (#GH132256)
 - Clang no longer crashes when establishing subsumption between some constraint expressions. (#GH122581)
 - Clang now issues an error when placement new is used to modify a const-qualified variable
   in a ``constexpr`` function. (#GH131432)

clang/lib/Sema/SemaDeclCXX.cpp

@@ -12182,10 +12182,14 @@ QualType Sema::BuildStdInitializerList(QualType Element, SourceLocation Loc) {
   Args.addArgument(TemplateArgumentLoc(TemplateArgument(Element),
                                        Context.getTrivialTypeSourceInfo(Element,
                                                                         Loc)));
+
+  QualType T = CheckTemplateIdType(TemplateName(StdInitializerList), Loc, Args);
+  if (T.isNull())
+    return QualType();
+
   return Context.getElaboratedType(
       ElaboratedTypeKeyword::None,
-      NestedNameSpecifier::Create(Context, nullptr, getStdNamespace()),
-      CheckTemplateIdType(TemplateName(StdInitializerList), Loc, Args));
+      NestedNameSpecifier::Create(Context, nullptr, getStdNamespace()), T);
 }
 
 bool Sema::isInitListConstructor(const FunctionDecl *Ctor) {

clang/test/SemaCXX/invalid-std-initializer-list.cpp

@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 %s -verify -std=c++20
+
+namespace std {
+
+template<class T, class = T::x> // expected-error 2 {{type 'int' cannot be used prior to '::' because it has no members}}
+class initializer_list;
+
+}
+
+namespace gh132256 {
+
+auto x = {1}; // expected-note {{in instantiation of default argument for 'initializer_list<int>' required here}}
+
+void f() {
+	for(int x : {1, 2}); // expected-note {{in instantiation of default argument for 'initializer_list<int>' required here}}
+}
+
+}