Disable -fdollars-in-identifiers by default

Clang used to enable -fdollars-in-identifiers by default for compatibility with GCC. However, this is no longer a conforming extension after WG21 P2558R2 and WG14 N2701. So this disables the dialect by default, which is a potentially breaking change for users. Note: some inline assembly constructs may use dollars in identifiers. We cannot enable the dialect mode automatically based on the user passing -fasm-blocks because that flag is implied by -fms-extensions which is enabled by default on Windows, and thus would mean we'd be enabling a non-conforming language extension by default on that platform. Users impacted by the change should explicitly add -fdollars-in-identifiers to their build scripts. Partially addresses #128939
llvm · AaronBallman · Apr 11, 2025 · Apr 11, 2025 · Apr 14, 2025 · Apr 15, 2025
commit c7e0132617ab01c12b393876b39381171996b793
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
@@ -48,6 +48,11 @@ C/C++ Language Potentially Breaking Changes
   ensure they are not caught by these optimizations.  It is also possible to use
   ``-fwrapv-pointer`` or   ``-fno-delete-null-pointer-checks`` to make pointer arithmetic
   on null pointers well-defined. (#GH130734, #GH130742, #GH130952)
+- Use of the dollar sign (``$``) in an identifier is no longer a conforming
+  extension in either C or C++, so ``-fdollars-in-identifiers`` is no longer
+  enabled by default. Use of the dollar sign in an identifier will now be
+  diagnosed as an error unless ``-fdollars-in-identifiers`` is explicitly
+  enabled.
 
 C++ Specific Potentially Breaking Changes
 -----------------------------------------

diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
@@ -119,7 +119,7 @@ LANGOPT(WChar             , 1, 0, "wchar_t keyword")
 LANGOPT(Char8             , 1, 0, "char8_t keyword")
 LANGOPT(IEEE128           , 1, 0, "__ieee128 keyword")
 LANGOPT(DeclSpecKeyword   , 1, 0, "__declspec keyword")
-BENIGN_LANGOPT(DollarIdents   , 1, 1, "'$' in identifiers")
+BENIGN_LANGOPT(DollarIdents   , 1, 0, "'$' in identifiers")
 BENIGN_LANGOPT(AsmPreprocessor, 1, 0, "preprocessor in asm mode")
 LANGOPT(GNUMode           , 1, 1, "GNU extensions")
 LANGOPT(GNUKeywords       , 1, 1, "GNU keywords")

diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
@@ -2088,7 +2088,7 @@ def fno_discard_value_names : Flag<["-"], "fno-discard-value-names">,
   Group<f_clang_Group>, Visibility<[ClangOption, DXCOption]>,
   HelpText<"Do not discard value names in LLVM IR">;
 defm dollars_in_identifiers : BoolFOption<"dollars-in-identifiers",
-  LangOpts<"DollarIdents">, Default<!strconcat("!", asm_preprocessor.KeyPath)>,
+  LangOpts<"DollarIdents">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption], "Allow">,
   NegFlag<SetFalse, [], [ClangOption], "Disallow">,
   BothFlags<[], [ClangOption, CC1Option], " '$' in identifiers">>;

diff --git a/clang/test/AST/ByteCode/codegen.m b/clang/test/AST/ByteCode/codegen.m
@@ -3,7 +3,7 @@
 /// See test/CodeGenObjC/constant-strings.m
 /// Test that we let the APValue we create for ObjCStringLiterals point to the right expression.
 
-// RUN: %clang_cc1 -triple x86_64-macho -emit-llvm -o %t %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -triple x86_64-macho -fdollars-in-identifiers  -emit-llvm -o %t %s -fexperimental-new-constant-interpreter
 // RUN: FileCheck --check-prefix=CHECK-NEXT < %t %s
 
 // Check that we set alignment 1 on the string.

diff --git a/clang/test/C/drs/dr0xx.c b/clang/test/C/drs/dr0xx.c
@@ -1,9 +1,9 @@
-/* RUN: %clang_cc1 -std=c89 -fsyntax-only -verify=expected,c89only -pedantic -Wno-declaration-after-statement -Wno-c11-extensions %s
-   RUN: %clang_cc1 -std=c89 -fsyntax-only -verify=expected,c89only -pedantic -Wno-declaration-after-statement -Wno-c11-extensions -fno-signed-char %s
-   RUN: %clang_cc1 -std=c99 -fsyntax-only -verify=expected,c99untilc2x -pedantic -Wno-c11-extensions %s
-   RUN: %clang_cc1 -std=c11 -fsyntax-only -verify=expected,c99untilc2x -pedantic %s
-   RUN: %clang_cc1 -std=c17 -fsyntax-only -verify=expected,c99untilc2x -pedantic %s
-   RUN: %clang_cc1 -std=c2x -fsyntax-only -verify=expected,c2xandup -pedantic %s
+/* RUN: %clang_cc1 -std=c89 -fdollars-in-identifiers -fsyntax-only -verify=expected,c89only -pedantic -Wno-declaration-after-statement -Wno-c11-extensions %s
+   RUN: %clang_cc1 -std=c89 -fdollars-in-identifiers -fsyntax-only -verify=expected,c89only -pedantic -Wno-declaration-after-statement -Wno-c11-extensions -fno-signed-char %s
+   RUN: %clang_cc1 -std=c99 -fdollars-in-identifiers -fsyntax-only -verify=expected,c99untilc2x -pedantic -Wno-c11-extensions %s
+   RUN: %clang_cc1 -std=c11 -fdollars-in-identifiers -fsyntax-only -verify=expected,c99untilc2x -pedantic %s
+   RUN: %clang_cc1 -std=c17 -fdollars-in-identifiers -fsyntax-only -verify=expected,c99untilc2x -pedantic %s
+   RUN: %clang_cc1 -std=c2x -fdollars-in-identifiers -fsyntax-only -verify=expected,c2xandup -pedantic %s
  */
 
 /* The following are DRs which do not require tests to demonstrate

diff --git a/clang/test/CodeGen/ms-inline-asm.c b/clang/test/CodeGen/ms-inline-asm.c
@@ -1,5 +1,5 @@
 // REQUIRES: x86-registered-target
-// RUN: %clang_cc1 %s -triple i386-apple-darwin10 -fasm-blocks -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 %s -triple i386-apple-darwin10 -fdollars-in-identifiers -fasm-blocks -emit-llvm -o - | FileCheck %s
 
 void t1(void) {
 // CHECK: @t1

diff --git a/clang/test/CodeGenObjC/extern-void-class-decl.m b/clang/test/CodeGenObjC/extern-void-class-decl.m
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple x86_64-apple-macosx10.14.0 %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.14.0 -fdollars-in-identifiers %s -emit-llvm -o - | FileCheck %s
 
 extern void OBJC_CLASS_$_f;
 Class c = (Class)&OBJC_CLASS_$_f;

diff --git a/clang/test/Lexer/dollar-idents.c b/clang/test/Lexer/dollar-idents.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -dump-tokens %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -dump-tokens -fdollars-in-identifiers %s 2>&1 | FileCheck %s
 // RUN: %clang_cc1 -dump-tokens -x assembler-with-cpp %s 2>&1 | FileCheck %s --check-prefix=CHECK-ASM
 // PR3808
 

diff --git a/clang/test/Lexer/gh128939.cpp b/clang/test/Lexer/gh128939.cpp
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -E -fdollars-in-identifiers %s 2>&1 | FileCheck %s --check-prefix=CHECK-DOLLARS
+// RUN: %clang_cc1 -E %s 2>&1 | FileCheck %s --check-prefix=CHECK-NO-DOLLARS
+// GH128939
+
+#define FOO$ 10
+#define STR(x) #x
+#define STR2(x) STR(x)
+const char *p = STR2(FOO$);
+
+// CHECK-NO-DOLLARS: const char *p = "$ 10$";
+// CHECK-DOLLARS: const char *p = "10";
+
+#define STR3 STR(
+const char *q = STR3$10);
+
+// CHECK-NO-DOLLARS: const char *q = "$10";
+// CHECK-DOLLARS: const char *q = STR3$10);
diff --git a/clang/test/Preprocessor/c90.c b/clang/test/Preprocessor/c90.c
@@ -7,7 +7,7 @@
 #define foo`bar   /* expected-error {{whitespace required after macro name}} */
 #define foo2!bar  /* expected-warning {{whitespace recommended after macro name}} */
 
-#define foo3$bar  /* expected-error {{'$' in identifier}} */
+#define foo3$bar  /* expected-error {{whitespace required after macro name}} */
 
 /* CHECK-NOT: this comment should be missing
  * CHECK: {{^}}// this comment should be present{{$}}