Fix PR34668 - P0704R1 implementation is too permissive

Summary:
https://bugs.llvm.org/show_bug.cgi?id=34668

Pretty straightforward.

Reviewers: rsmith, Rakete1111

Reviewed By: Rakete1111

Subscribers: Rakete1111, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D38075

llvm-svn: 337017
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 8ed795a..fbf572e 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -5472,8 +5472,9 @@
 
     case RQ_LValue:
       if (!isIndirect && !LHS.get()->Classify(Context).isLValue()) {
-        // C++2a allows functions with ref-qualifier & if they are also 'const'.
-        if (Proto->isConst())
+        // C++2a allows functions with ref-qualifier & if their cv-qualifier-seq
+        // is (exactly) 'const'.
+        if (Proto->isConst() && !Proto->isVolatile())
           Diag(Loc, getLangOpts().CPlusPlus2a
                         ? diag::warn_cxx17_compat_pointer_to_const_ref_member_on_rvalue
                         : diag::ext_pointer_to_const_ref_member_on_rvalue);