https://github.com/AaronBallman created
https://github.com/llvm/llvm-project/pull/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.
>From 3471e09bd9a871216408336aa86a3b24c6e433a1 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aa...@aaronballman.com>
Date: Wed, 30 Apr 2025 14:33:03 -0400
Subject: [PATCH] [C] Add -Wduplicate-decl-specifier to -Wc++-compat
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.
---
clang/docs/ReleaseNotes.rst | 3 +++
clang/include/clang/Basic/DiagnosticGroups.td | 6 ++++--
clang/test/Sema/warn-duplicate-decl-specifier.c | 17 +++++++++++++++++
3 files changed, 24 insertions(+), 2 deletions(-)
create mode 100644 clang/test/Sema/warn-duplicate-decl-specifier.c
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 5cc1a36fac1e8..3becd3e3e4603 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td
b/clang/include/clang/Basic/DiagnosticGroups.td
index 75e8fc541305b..de3374962c6b0 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/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">;
diff --git a/clang/test/Sema/warn-duplicate-decl-specifier.c
b/clang/test/Sema/warn-duplicate-decl-specifier.c
new file mode 100644
index 0000000000000..d57bb867f66f1
--- /dev/null
+++ b/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;
+
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