PR27037: Use correct CVR qualifier on an upcast on method pointer call

[tzik]

Summary

Clang currently ignores a const qualifier of a method receiver when the method is invoked through
a method pointer, and the invocation implies an implicit cast to the parent class, which hurts the
const correctness. This patch fixes it by restoring the qualifier on the parent class from the child class.

[tzik]

Hi, Richard.
Could you PTAL to this?

[rsmith]

lib/Sema/SemaExprCXX.cpp

5108

In the "indirect" case, the cv-qualifiers should be taken from the pointee type of the LHS and applied to the pointee type of UseType. I believe this patch will not be enough to cause us to reject the indirect version of your testcase:

((&b)->*&B::f)();  // expected-error{{drops 'const' qualifier}}

Moreover, we should be preserving all qualifiers, not just cvr-qualifiers; for example, this should also preserve the address space.

[tzik]

Cover indirect case and non-CVR qualifiers

[tzik]

lib/Sema/SemaExprCXX.cpp

5108

Make sense. OK, I updated the CL to cover that cases.
Could you take another look?

[rsmith]

Looks good to me, thanks! Do you need someone to commit this for you?

[tzik]

In D33875#774293, @rsmith wrote:

Looks good to me, thanks! Do you need someone to commit this for you?

Yes, could you commit this?

lib/Sema/SemaExprCXX.cpp

if (!Context.hasSameUnqualifiedType(Class, LHSType)) {
     CXXCastPath BasePath;
     if (CheckDerivedToBaseConversion(LHSType, Class, Loc,
                                      SourceRange(LHS.get()->getLocStart(),
                                                  RHS.get()->getLocEnd()),
                                      &BasePath))
       return QualType();
 
     // Cast LHS to type of use.
-    QualType UseType = isIndirect ? Context.getPointerType(Class) : Class;
+    QualType UseType = Context.getQualifiedType(Class, LHSType.getQualifiers());
+    if (isIndirect)
+      UseType = Context.getPointerType(UseType);
     ExprValueKind VK = isIndirect ? VK_RValue : LHS.get()->getValueKind();
     LHS = ImpCastExprToType(LHS.get(), UseType, CK_DerivedToBase, VK,
                             &BasePath);
   }
 
   if (isa<CXXScalarValueInitExpr>(RHS.get()->IgnoreParens())) {
     // Diagnose use of pointer-to-member type which when used as
     // the functional cast in a pointer-to-member expression.

test/SemaCXX/PR27037.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+struct A {
+  void f();
+};
+
+struct B : A {};
+
+void m() {
+  const B b;
+  (b.*&B::f)();  // expected-error{{drops 'const' qualifier}}
+  ((&b)->*&B::f)();  // expected-error{{drops 'const' qualifier}}
+}