[clang] [PATCH] [clang][frontend] Fix AllocKind retrieving for `CXXConstructorDecl` refs #132794 (PR #133077)

[via cfe-commits]

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@@ -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;
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cor3ntin wrote:

This change does not look quite correct.
Indeed `numTrailingObjects` is an internal detail of the object that should preserve its invariants. i.e, it is exactly whether an explicit specifier is **stored** as a trailing object of this object.

However, you are most probably on the right path.
In `ASTWriterDecl`, `addExplicitSpecifier(D->getExplicitSpecifier(), Record);` does indeed look incorrect. Because again, we are trying to serialize that object  - not parts that are stored in an other canonicl decl.

I think the solution might be as simple as replacing that line with `addExplicitSpecifier(D->getExplicitSpecifierInternal(), Record);` (`getExplicitSpecifierInternal()` does not call `getCanonicalDecl()`) 

https://github.com/llvm/llvm-project/pull/133077