[clang] Warn about deprecated volatile-qualified return types (#137899)

The old codepath in GetFullTypeForDeclarator was under "if (not a class type)"
so that it failed to warn for class types. Move the diagnostic outside
of the "if" so that it warns in the proper situations.

Fixes #133380

Co-authored-by: cor3ntin <corentinjabot@gmail.com>

clang/docs/ReleaseNotes.rst

@@ -509,6 +509,9 @@ Improvements to Clang's diagnostics
 - Clang now prints the namespace for an attribute, if any,
   when emitting an unknown attribute diagnostic.
 
+- ``-Wvolatile`` now warns about volatile-qualified class return types
+  as well as volatile-qualified scalar return types. Fixes #GH133380
+
 - Several compatibility diagnostics that were incorrectly being grouped under
   ``-Wpre-c++20-compat`` are now part of ``-Wc++20-compat``. (#GH138775)
 
@@ -921,7 +924,8 @@ Improvements
 ^^^^^^^^^^^^
 
 Additional Information
-======================
+
+===================
 
 A wide variety of additional information is available on the `Clang web
 page <https://clang.llvm.org/>`_. The web page contains versions of the

clang/lib/Sema/SemaType.cpp

@@ -5056,13 +5056,13 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
           S.Diag(DeclType.Loc, diag::err_func_returning_qualified_void) << T;
         } else
           diagnoseRedundantReturnTypeQualifiers(S, T, D, chunkIndex);
-
-        // C++2a [dcl.fct]p12:
-        //   A volatile-qualified return type is deprecated
-        if (T.isVolatileQualified() && S.getLangOpts().CPlusPlus20)
-          S.Diag(DeclType.Loc, diag::warn_deprecated_volatile_return) << T;
       }
 
+      // C++2a [dcl.fct]p12:
+      //   A volatile-qualified return type is deprecated
+      if (T.isVolatileQualified() && S.getLangOpts().CPlusPlus20)
+        S.Diag(DeclType.Loc, diag::warn_deprecated_volatile_return) << T;
+
       // Objective-C ARC ownership qualifiers are ignored on the function
       // return type (by type canonicalization). Complain if this attribute
       // was written here.

clang/test/CXX/expr/expr.const/p2-0x.cpp

@@ -356,7 +356,7 @@ namespace LValueToRValue {
   // - a non-volatile glvalue of literal type that refers to a non-volatile
   //   temporary object whose lifetime has not ended, initialized with a
   //   constant expression;
-  constexpr volatile S f() { return S(); }
+  constexpr volatile S f() { return S(); } // cxx20-warning {{volatile-qualified return type 'volatile S' is deprecated}}
   static_assert(f().i, ""); // expected-error {{constant expression}} expected-note {{read of volatile-qualified type}}
   static_assert(((volatile const S&&)(S)0).i, ""); // expected-error {{constant expression}} expected-note {{read of volatile-qualified type}}
 }

clang/test/SemaCXX/deprecated.cpp

@@ -231,6 +231,13 @@ namespace DeprecatedVolatile {
     a = c = a;
     b += a;
   }
+
+  volatile struct amber jurassic();
+    // cxx20-warning@-1 {{volatile-qualified return type 'volatile struct amber' is deprecated}}
+  void trex(volatile short left_arm, volatile struct amber right_arm);
+    // cxx20-warning@-1 {{volatile-qualified parameter type 'volatile short' is deprecated}}
+    // cxx20-warning@-2 {{volatile-qualified parameter type 'volatile struct amber' is deprecated}}
+  void fly(volatile struct pterosaur* pteranodon);
 }
 
 namespace ArithConv {