Fix nullptr dereference found by Coverity static analysis tool

[Manna]

Summary

Reported by Coverity:

In clang::​ASTContext::​hasUniqueObjectRepresentations(clang::​QualType, bool): Return value of function which returns null is dereferenced without checking.

(Ty->isMemberPointerType()) {
    	//returned_null: getAs returns nullptr.
    	//var_assigned: Assigning: MPT = nullptr return value from getAs.
    const auto *MPT = Ty->getAs<MemberPointerType>();
    	
   //dereference: Dereferencing a pointer that might be nullptr MPT when calling getMemberPointerInfo. (The virtual call resolves to 
   <unnamed>::ItaniumCXXABI::getMemberPointerInfo.)
    return !ABI->getMemberPointerInfo(MPT).HasPadding;
  }

This patch uses castAs instead of getAs.

[erichkeane]

Looks like both ABIs that use this assume the parameter passed to getMemberPointerInfo is non-null. While we DO check the type (in reality, we probably should be doing a if (const auto *MPT = Ty->getAs<MemberPointerType>()) instead, I think this is a problem that at least SHOULD be fixed somehow.

Why was this abandoned?

[Manna]

In D149922#4321972, @erichkeane wrote:

Looks like both ABIs that use this assume the parameter passed to getMemberPointerInfo is non-null. While we DO check the type (in reality, we probably should be doing a if (const auto *MPT = Ty->getAs<MemberPointerType>()) instead, I think this is a problem that at least SHOULD be fixed somehow.

Why was this abandoned?

@erichkeane, Sorry i abandoned the patch without any comment. My change (use castAs instead of getAs) was not correct.

Looks like both ABIs that use this assume the parameter passed to getMemberPointerInfo is non-null. While we DO check the type (in reality, we probably should be doing a if (const auto *MPT = Ty->getAs<MemberPointerType>()) instead

Agreed.

[Manna]

I have updated patch

[erichkeane]

clang/lib/AST/ASTContext.cpp

2863

Remove the curley braces, otherwise LGTM.

[Manna]

clang/lib/AST/ASTContext.cpp

2863

Missed that. Done

[Manna]

Thanks @erichkeane for reviews!

clang/lib/AST/ASTContext.cpp

if (Ty->isIntegralOrEnumerationType()) {
     return true;
   }
 
   // All other pointers are unique.
   if (Ty->isPointerType())
     return true;
 
   if (Ty->isMemberPointerType()) {
-    const auto *MPT = Ty->getAs<MemberPointerType>();
+    const auto *MPT = Ty->castAs<MemberPointerType>();
     return !ABI->getMemberPointerInfo(MPT).HasPadding;
   }
 
   if (Ty->isRecordType()) {
     const RecordDecl *Record = Ty->castAs<RecordType>()->getDecl();
 
     if (Record->isInvalidDecl())
       return false;