[C] Add -Wduplicate-decl-specifier to -Wc++-compat #138012
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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.
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@llvm/pr-subscribers-clang Author: Aaron Ballman (AaronBallman) ChangesThe existing diagnostic, already enabled by default in C, will diagnose use of duplicate declaration specifiers (e.g., 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. Full diff: https://github.com/llvm/llvm-project/pull/138012.diff 3 Files Affected:
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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erichkeane
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Apr 30, 2025
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/143/builds/7381 Here is the relevant piece of the build log for the reference |
shafik
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May 2, 2025
IanWood1
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May 6, 2025
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.
GeorgeARM
pushed a commit
to GeorgeARM/llvm-project
that referenced
this pull request
May 7, 2025
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.
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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.