[Clang] emit -Wignored-qualifiers diagnostic for cv-qualified base classes

clang/docs/ReleaseNotes.rst

@@ -733,6 +733,7 @@ Improvements to Clang's diagnostics
       scope.Unlock();
       require(scope); // Warning!  Requires mu1.
     }
+- Clang now emits a ``-Wignored-qualifiers`` diagnostic when a base class includes cv-qualifiers (#GH55474).
 
 Improvements to Clang's time-trace
 ----------------------------------

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -487,6 +487,10 @@ def err_noreturn_non_function : Error<
 def warn_qual_return_type : Warning<
   "'%0' type qualifier%s1 on return type %plural{1:has|:have}1 no effect">,
   InGroup<IgnoredQualifiers>, DefaultIgnore;
+def warn_qual_base_type : Warning<
+  "'%0' qualifier%s1 on base class type %2 have no effect">,
+  InGroup<IgnoredQualifiers>, DefaultIgnore;
+
 def warn_deprecated_redundant_constexpr_static_def : Warning<
   "out-of-line definition of constexpr static data member is redundant "
   "in C++17 and is deprecated">,

clang/lib/Sema/SemaDeclCXX.cpp

@@ -2655,6 +2655,15 @@ CXXBaseSpecifier *Sema::CheckBaseSpecifier(CXXRecordDecl *Class,
       return nullptr;
     }
 
+    if (BaseType.hasQualifiers() && !isa<SubstTemplateTypeParmType>(BaseType)) {
+      std::string Quals =
+          BaseType.getQualifiers().getAsString(Context.getPrintingPolicy());
+      Diag(BaseLoc, diag::warn_qual_base_type)
+          << Quals << std::count(Quals.begin(), Quals.end(), ' ') + 1
+          << BaseType;
+      Diag(BaseLoc, diag::note_base_class_specified_here) << BaseType;
+    }
+
     // For the MS ABI, propagate DLL attributes to base class templates.
     if (Context.getTargetInfo().getCXXABI().isMicrosoft() ||
         Context.getTargetInfo().getTriple().isPS()) {

clang/test/SemaCXX/warn-base-type-qualifiers.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 %s -std=c++11 -Wignored-qualifiers -verify
+
+class A { };
+
+typedef const A A_Const;
+class B : public A_Const { }; // expected-warning {{'const' qualifier on base class type 'A_Const' (aka 'const A') have no effect}} \
+                              // expected-note {{base class 'A_Const' (aka 'const A') specified here}}
+
+typedef const volatile A A_Const_Volatile;
+class C : public A_Const_Volatile { }; // expected-warning {{'const volatile' qualifiers on base class type 'A_Const_Volatile' (aka 'const volatile A') have no effect}} \
+                                       // expected-note {{base class 'A_Const_Volatile' (aka 'const volatile A') specified here}}
+
+struct D {
+  D(int);
+};
+template <typename T> struct E : T {
+  using T::T;
+  E(int &) : E(0) {}
+};
+E<const D> e(1);