[PATCH] [clang][frontend] Fix AllocKind retrieval for CXXConstructorDecl (refs #132794)

When retrieving the ExplicitSpecifier from a CXXConstructorDecl, one
of its canonical declarations is returned. To correctly write the
declaration record with the ExplicitSpecifier, the AllocKind of that
canonical declaration must be used.

Failing to do so results in a crash during deserialization.

Author: koplas

clang/include/clang/AST/DeclCXX.h

@@ -2601,10 +2601,11 @@ class CXXConstructorDecl final
   void anchor() override;
 
   size_t numTrailingObjects(OverloadToken<InheritedConstructor>) const {
-    return CXXConstructorDeclBits.IsInheritingConstructor;
+    return getCanonicalDecl()->CXXConstructorDeclBits.IsInheritingConstructor;
   }
   size_t numTrailingObjects(OverloadToken<ExplicitSpecifier>) const {
-    return CXXConstructorDeclBits.HasTrailingExplicitSpecifier;
+    return getCanonicalDecl()
+        ->CXXConstructorDeclBits.HasTrailingExplicitSpecifier;
   }
 
   ExplicitSpecifier getExplicitSpecifierInternal() const {

clang/test/Modules/ComplexExplicitSpecifier.cpp

@@ -0,0 +1,45 @@
+// Tests complex explicit constructor across modules.
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/Foo.cppm \
+// RUN: -o %t/Foo.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface \
+// RUN: -fmodule-file=Foo=%t/Foo.pcm \
+// RUN: %t/Bar.cppm \
+// RUN: -o %t/Bar.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-obj \
+// RUN: -main-file-name Bar.cppm \
+// RUN: -fmodule-file=Foo=%t/Foo.pcm \
+// RUN: -x pcm %t/Bar.pcm \
+// RUN: -o %t/Bar.o
+
+//--- Foo.cppm
+export module Foo;
+
+export {
+template<class T>
+class Foo {
+  public:
+    template<class... Args>
+    explicit (sizeof...(Args) == 1) Foo(Args&&... args);
+};
+}
+
+template<class T>
+template<class... Args>
+inline Foo<T>::Foo(Args&&... args) {}
+
+//--- Bar.cppm
+export module Bar;
+import Foo;
+
+struct Bar {};
+
+void a() {
+  auto foo = Foo<Bar>{};
+}