[C] Add -Wduplicate-decl-specifier to -Wc++-compat (#138012)

The existing diagnostic, already enabled by default in C, will diagnose
use of duplicate declaration specifiers (e.g., `const const`). However,
the C++ standard claims that is ill-formed, so the diagnostic is now
also controlled via -Wc++-compat.

Note: Clang treats this as a warning in C++ rather than an error, but
GCC does treat this as an error in C++, so the compatibility concerns
are minor but do exist.

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -188,6 +188,9 @@ C Language Changes
   ``-Wunterminated-string-initialization``. However, this diagnostic is not
   silenced by the ``nonstring`` attribute as these initializations are always
   incompatible with C++.
+- Added the existing ``-Wduplicate-decl-specifier`` diagnostic, which is on by
+  default, to ``-Wc++-compat`` because duplicated declaration specifiers are
+  not valid in C++.
 
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticGroups.td

@@ -156,6 +156,8 @@ def BuiltinRequiresHeader : DiagGroup<"builtin-requires-header">;
 def C99Compat : DiagGroup<"c99-compat">;
 def C23Compat : DiagGroup<"c23-compat">;
 def : DiagGroup<"c2x-compat", [C23Compat]>;
+
+def DuplicateDeclSpecifier : DiagGroup<"duplicate-decl-specifier">;
 def InitStringTooLongMissingNonString :
   DiagGroup<"unterminated-string-initialization">;
 def InitStringTooLongForCpp :
@@ -168,7 +170,8 @@ def ImplicitIntToEnumCast : DiagGroup<"implicit-int-enum-cast",
                                       [ImplicitEnumEnumCast]>;
 def CXXCompat: DiagGroup<"c++-compat", [ImplicitVoidPtrCast, DefaultConstInit,
                                         ImplicitIntToEnumCast, HiddenCppDecl,
-                                        InitStringTooLongForCpp]>;
+                                        InitStringTooLongForCpp,
+                                        DuplicateDeclSpecifier]>;
 
 def ExternCCompat : DiagGroup<"extern-c-compat">;
 def KeywordCompat : DiagGroup<"keyword-compat">;
@@ -814,7 +817,6 @@ def TautologicalCompare : DiagGroup<"tautological-compare",
                                      TautologicalObjCBoolCompare,
                                      TautologicalNegationCompare]>;
 def HeaderHygiene : DiagGroup<"header-hygiene">;
-def DuplicateDeclSpecifier : DiagGroup<"duplicate-decl-specifier">;
 def CompareDistinctPointerType : DiagGroup<"compare-distinct-pointer-types">;
 def GNUUnionCast : DiagGroup<"gnu-union-cast">;
 def GNUVariableSizedTypeNotAtEnd : DiagGroup<"gnu-variable-sized-type-not-at-end">;

clang/test/Sema/warn-duplicate-decl-specifier.c

@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wduplicate-decl-specifier %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wc++-compat %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wno-duplicate-decl-specifier -Wc++-compat %s
+// RUN: %clang_cc1 -fsyntax-only -verify=good -Wc++-compat -Wno-duplicate-decl-specifier %s
+// RUN: %clang_cc1 -fsyntax-only -verify=good -Wno-duplicate-decl-specifier %s
+// RUN: %clang_cc1 -fsyntax-only -verify -x c++ %s
+// good-no-diagnostics
+
+// Note: we treat this as a warning in C++, so you get the same diagnostics in
+// either language mode. However, GCC diagnoses this as an error, so the
+// compatibility warning has value.
+const const int i = 12; // expected-warning {{duplicate 'const' declaration specifier}}
+
+__attribute__((address_space(1)))
+__attribute__((address_space(1))) // expected-warning {{multiple identical address spaces specified for type}}
+int j = 12;
+