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

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Merged
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May 1, 2025
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[C] Add -Wduplicate-decl-specifier to -Wc++-compat #138012

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3 changes: 3 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -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
^^^^^^^^^^^^^^^^^^^
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6 changes: 4 additions & 2 deletions clang/include/clang/Basic/DiagnosticGroups.td
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Expand Up @@ -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 :
Expand All @@ -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">;
Expand Down Expand Up @@ -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">;
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17 changes: 17 additions & 0 deletions clang/test/Sema/warn-duplicate-decl-specifier.c
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@@ -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;