https://github.com/yronglin created https://github.com/llvm/llvm-project/pull/218322
This PR implement [P2843R3 - Preprocessing is never undefined](https://wg21.link/P2843) `define`/`undef` keyword part. The two major changes in this patch: - Error-by-default warning for `#define` keyword. - Pedantic warning for `#undef` keyword. RFC: https://discourse.llvm.org/t/rfc-default-on-diagnostics-for-implement-p2843r3-preprocessing-is-never-undefined/90863 >From ac925ed27622ca13c93d09255044be33ce679744 Mon Sep 17 00:00:00 2001 From: yronglin <[email protected]> Date: Sun, 23 Aug 2026 22:41:55 -0700 Subject: [PATCH] [C++26] Implement P2843R3 - Preprocessing is never undefined (define/undef keyword part) Signed-off-by: yronglin <[email protected]> --- clang/docs/ReleaseNotes.md | 7 ++++ clang/include/clang/Basic/DiagnosticGroups.td | 3 +- .../include/clang/Basic/DiagnosticLexKinds.td | 5 ++- clang/lib/Lex/PPDirectives.cpp | 14 +++++-- clang/test/C/drs/dr1xx.c | 2 +- clang/test/CXX/cpp/cpp.replace.general/p9.cpp | 38 +++++++++++++++++++ clang/test/CXX/drs/cwg3088.cpp | 14 +++---- clang/test/Preprocessor/macro-reserved.c | 30 +++++++-------- clang/test/Preprocessor/macro-reserved.cpp | 28 +++++++------- .../test/Preprocessor/warn-macro-undef-true.c | 8 +--- clang/test/Sema/c++-keyword-in-c.c | 8 ++-- clang/test/SemaCUDA/noinline.cu | 2 +- clang/test/SemaCXX/abstract.cpp | 1 + clang/test/SemaCXX/consteval-builtin.cpp | 2 +- clang/test/SemaTemplate/instantiate-array.cpp | 2 +- clang/www/cxx_status.html | 14 ++++++- 16 files changed, 122 insertions(+), 56 deletions(-) create mode 100644 clang/test/CXX/cpp/cpp.replace.general/p9.cpp diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md index ca0dbfa2af229..f39be8ab74946 100644 --- a/clang/docs/ReleaseNotes.md +++ b/clang/docs/ReleaseNotes.md @@ -416,6 +416,13 @@ features cannot lower the translation-unit ABI level; - `-Wc++98-compat` now diagnoses explicit conversion functions in C++20 and later, matching the behavior in C++11 through C++17. (#GH161689) +- `-Wkeyword-macro` is now enabled by default for `#define` directives whose + macro name is a keyword or an identifier with special meaning. The + corresponding `#undef` diagnostic is not enabled by default; it is enabled by + `-pedantic`, `-Wkeyword-macro`, or its more specific + `-Wkeyword-macro-undef` subgroup. + + ### Improvements to Clang's time-trace ### Improvements to Coverage Mapping diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td index 9ee0b61a96a32..d5ef1270bf4bc 100644 --- a/clang/include/clang/Basic/DiagnosticGroups.td +++ b/clang/include/clang/Basic/DiagnosticGroups.td @@ -1127,7 +1127,8 @@ def : DiagGroup<"sequence-point", [Unsequenced]>; // Preprocessor warnings. def AmbiguousMacro : DiagGroup<"ambiguous-macro">; -def KeywordAsMacro : DiagGroup<"keyword-macro">; +def KeywordAsMacroUndef : DiagGroup<"keyword-macro-undef">; +def KeywordAsMacro : DiagGroup<"keyword-macro", [KeywordAsMacroUndef]>; def ReservedIdAsMacro : DiagGroup<"reserved-macro-identifier">; def ReservedIdAsMacroAlias : DiagGroup<"reserved-id-macro", [ReservedIdAsMacro]>; def ReservedAttributeIdentifier : DiagGroup<"reserved-attribute-identifier">; diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td index bdb78ed80b7b4..49013756b2dfd 100644 --- a/clang/include/clang/Basic/DiagnosticLexKinds.td +++ b/clang/include/clang/Basic/DiagnosticLexKinds.td @@ -435,8 +435,11 @@ def note_pp_ambiguous_macro_chosen : Note< "expanding this definition of %0">; def note_pp_ambiguous_macro_other : Note< "other definition of %0">; -def warn_pp_macro_hides_keyword : Extension< +def warn_pp_macro_hides_keyword : ExtWarn< "keyword is hidden by macro definition">, InGroup<KeywordAsMacro>; +def ext_pp_macro_name_is_keyword : Extension< + "keyword or identifier with special meaning is used as a macro name">, + InGroup<KeywordAsMacroUndef>; def warn_pp_macro_is_reserved_id : Warning< "macro name is a reserved identifier">, DefaultIgnore, InGroup<ReservedIdAsMacro>; diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp index 29236bd0f4782..5bca4f4ce6578 100644 --- a/clang/lib/Lex/PPDirectives.cpp +++ b/clang/lib/Lex/PPDirectives.cpp @@ -103,8 +103,10 @@ SourceRange Preprocessor::DiscardUntilEndOfDirective( /// Enumerates possible cases of #define/#undef a reserved identifier. enum MacroDiag { - MD_NoWarn, //> Not a reserved identifier - MD_KeywordDef, //> Macro hides keyword, enabled by default + MD_NoWarn, //> Not a reserved identifier + MD_KeywordDef, //> Macro hides keyword, enabled by default + MD_KeywordUnDef, //> Undef keyword, It is generally harmless and widely used, + //> enabled in pedantic mode. MD_ReservedMacro, //> #define of #undef reserved id, disabled by default MD_ReservedAttributeIdentifier }; @@ -213,7 +215,11 @@ static MacroDiag shouldWarnOnMacroDef(Preprocessor &PP, IdentifierInfo *II) { static MacroDiag shouldWarnOnMacroUndef(Preprocessor &PP, IdentifierInfo *II) { const LangOptions &Lang = PP.getLangOpts(); - // Do not warn on keyword undef. It is generally harmless and widely used. + StringRef Text = II->getName(); + if (II->isKeyword(Lang)) + return MD_KeywordUnDef; + if (Lang.CPlusPlus11 && (Text == "override" || Text == "final")) + return MD_KeywordUnDef; if (isReservedInAllContexts(II->isReserved(Lang))) return MD_ReservedMacro; if (isReservedCXXAttributeName(PP, II)) @@ -412,6 +418,8 @@ bool Preprocessor::CheckMacroName(Token &MacroNameTok, MacroUse isDefineUndef, if (ShadowFlag) *ShadowFlag = true; } + if (D == MD_KeywordUnDef) + Diag(MacroNameTok, diag::ext_pp_macro_name_is_keyword); if (D == MD_ReservedMacro) Diag(MacroNameTok, diag::warn_pp_macro_is_reserved_id); if (D == MD_ReservedAttributeIdentifier) diff --git a/clang/test/C/drs/dr1xx.c b/clang/test/C/drs/dr1xx.c index 055a30cc9c4b8..1704aa0a361d3 100644 --- a/clang/test/C/drs/dr1xx.c +++ b/clang/test/C/drs/dr1xx.c @@ -160,7 +160,7 @@ void dr106(void *p, int i) { void dr108(void) { #define const const int i = 12; -#undef const +#undef const /* expected-warning {{keyword or identifier with special meaning is used as a macro name}} */ const int j = 12; /* expected-note {{variable 'j' declared const here}} */ i = 100; /* Okay, the keyword was hidden by the macro. */ diff --git a/clang/test/CXX/cpp/cpp.replace.general/p9.cpp b/clang/test/CXX/cpp/cpp.replace.general/p9.cpp new file mode 100644 index 0000000000000..7d775939063a2 --- /dev/null +++ b/clang/test/CXX/cpp/cpp.replace.general/p9.cpp @@ -0,0 +1,38 @@ +// RUN: %clang_cc1 -std=c++26 -fsyntax-only -verify=default %s +// RUN: %clang_cc1 -std=c++26 -pedantic -fsyntax-only -verify=pedantic %s +// RUN: %clang_cc1 -std=c++26 -pedantic -Wno-keyword-macro-undef -fsyntax-only -verify=default %s +// RUN: %clang_cc1 -std=c++26 -pedantic-errors -fsyntax-only -verify=pedantic-errors %s + +// [cpp.replace.general]/p9: A translation unit shall not #define or #undef +// macro names lexically identical to keywords ([lex.key]) or to the identifiers +// listed in Table 4. +#define for 0 +// default-warning@-1 {{keyword is hidden by macro definition}} +// pedantic-warning@-2 {{keyword is hidden by macro definition}} +// pedantic-errors-error@-3 {{keyword is hidden by macro definition}} +#undef for +// pedantic-warning@-1 {{keyword or identifier with special meaning is used as a macro name}} +// pedantic-errors-error@-2 {{keyword or identifier with special meaning is used as a macro name}} + +#define final 1 +// default-warning@-1 {{keyword is hidden by macro definition}} +// pedantic-warning@-2 {{keyword is hidden by macro definition}} +// pedantic-errors-error@-3 {{keyword is hidden by macro definition}} +#undef final +// pedantic-warning@-1 {{keyword or identifier with special meaning is used as a macro name}} +// pedantic-errors-error@-2 {{keyword or identifier with special meaning is used as a macro name}} + +#define override +// default-warning@-1 {{keyword is hidden by macro definition}} +// pedantic-warning@-2 {{keyword is hidden by macro definition}} +// pedantic-errors-error@-3 {{keyword is hidden by macro definition}} +#undef override +// pedantic-warning@-1 {{keyword or identifier with special meaning is used as a macro name}} +// pedantic-errors-error@-2 {{keyword or identifier with special meaning is used as a macro name}} + +// Empty definitions of qualifier keywords are accepted for compatibility with +// configuration scripts, but #undef is still diagnosed in pedantic modes. +#define const +#undef const +// pedantic-warning@-1 {{keyword or identifier with special meaning is used as a macro name}} +// pedantic-errors-error@-2 {{keyword or identifier with special meaning is used as a macro name}} diff --git a/clang/test/CXX/drs/cwg3088.cpp b/clang/test/CXX/drs/cwg3088.cpp index 04333d8f91a46..39c02dd31fc6c 100644 --- a/clang/test/CXX/drs/cwg3088.cpp +++ b/clang/test/CXX/drs/cwg3088.cpp @@ -1,10 +1,10 @@ -// RUN: %clang_cc1 -std=c++98 -fexceptions -fcxx-exceptions -pedantic-errors %s -verify-directives -verify=expected -// RUN: %clang_cc1 -std=c++11 -fexceptions -fcxx-exceptions -pedantic-errors %s -verify-directives -verify=expected,since-cxx11 -// RUN: %clang_cc1 -std=c++14 -fexceptions -fcxx-exceptions -pedantic-errors %s -verify-directives -verify=expected,since-cxx11 -// RUN: %clang_cc1 -std=c++17 -fexceptions -fcxx-exceptions -pedantic-errors %s -verify-directives -verify=expected,since-cxx11 -// RUN: %clang_cc1 -std=c++20 -fexceptions -fcxx-exceptions -pedantic-errors %s -verify-directives -verify=expected,since-cxx11,since-cxx20 -// RUN: %clang_cc1 -std=c++23 -fexceptions -fcxx-exceptions -pedantic-errors %s -verify-directives -verify=expected,since-cxx11,since-cxx20 -// RUN: %clang_cc1 -std=c++2c -fexceptions -fcxx-exceptions -pedantic-errors %s -verify-directives -verify=expected,since-cxx11,since-cxx20 +// RUN: %clang_cc1 -std=c++98 -fexceptions -fcxx-exceptions -pedantic-errors -Wno-keyword-macro-undef %s -verify-directives -verify=expected +// RUN: %clang_cc1 -std=c++11 -fexceptions -fcxx-exceptions -pedantic-errors -Wno-keyword-macro-undef %s -verify-directives -verify=expected,since-cxx11 +// RUN: %clang_cc1 -std=c++14 -fexceptions -fcxx-exceptions -pedantic-errors -Wno-keyword-macro-undef %s -verify-directives -verify=expected,since-cxx11 +// RUN: %clang_cc1 -std=c++17 -fexceptions -fcxx-exceptions -pedantic-errors -Wno-keyword-macro-undef %s -verify-directives -verify=expected,since-cxx11 +// RUN: %clang_cc1 -std=c++20 -fexceptions -fcxx-exceptions -pedantic-errors -Wno-keyword-macro-undef %s -verify-directives -verify=expected,since-cxx11,since-cxx20 +// RUN: %clang_cc1 -std=c++23 -fexceptions -fcxx-exceptions -pedantic-errors -Wno-keyword-macro-undef %s -verify-directives -verify=expected,since-cxx11,since-cxx20 +// RUN: %clang_cc1 -std=c++2c -fexceptions -fcxx-exceptions -pedantic-errors -Wno-keyword-macro-undef %s -verify-directives -verify=expected,since-cxx11,since-cxx20 // The list of keywords was taken fron [lex.key] for C++98, and the difference // between [lex.key] contents in N and N-1 revisions of the Standard otherwise. diff --git a/clang/test/Preprocessor/macro-reserved.c b/clang/test/Preprocessor/macro-reserved.c index 6026a9f60730e..875fce19513c6 100644 --- a/clang/test/Preprocessor/macro-reserved.c +++ b/clang/test/Preprocessor/macro-reserved.c @@ -9,7 +9,7 @@ #define __STDC__ 1 // expected-warning {{redefining builtin macro}} #define __clang__ 1 -#undef for +#undef for // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #undef final #undef __HAVE_X #undef __cplusplus @@ -30,30 +30,30 @@ #define extern #define inline -#undef while -#undef const -#undef static -#undef extern -#undef inline +#undef while // expected-warning {{keyword or identifier with special meaning is used as a macro name}} +#undef const // expected-warning {{keyword or identifier with special meaning is used as a macro name}} +#undef static // expected-warning {{keyword or identifier with special meaning is used as a macro name}} +#undef extern // expected-warning {{keyword or identifier with special meaning is used as a macro name}} +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define inline __inline -#undef inline +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define inline __inline__ -#undef inline +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define inline inline__ // expected-warning {{keyword is hidden by macro definition}} -#undef inline +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define extern __inline // expected-warning {{keyword is hidden by macro definition}} -#undef extern +#undef extern // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define extern __extern // expected-warning {{keyword is hidden by macro definition}} -#undef extern +#undef extern // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define extern __extern__ // expected-warning {{keyword is hidden by macro definition}} -#undef extern +#undef extern // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define inline _inline // expected-warning {{keyword is hidden by macro definition}} -#undef inline +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define volatile // expected-warning {{keyword is hidden by macro definition}} -#undef volatile +#undef volatile // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #pragma clang diagnostic warning "-Wreserved-macro-identifier" @@ -64,7 +64,7 @@ #define _HAVE_X 0 // expected-warning {{macro name is a reserved identifier}} #define X__Y -#undef switch +#undef switch // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #undef final #undef __cplusplus // expected-warning {{macro name is a reserved identifier}} #undef _HAVE_X // expected-warning {{macro name is a reserved identifier}} diff --git a/clang/test/Preprocessor/macro-reserved.cpp b/clang/test/Preprocessor/macro-reserved.cpp index 53bb3634bac4c..a90966dc180dd 100644 --- a/clang/test/Preprocessor/macro-reserved.cpp +++ b/clang/test/Preprocessor/macro-reserved.cpp @@ -6,7 +6,7 @@ #define _HAVE_X 0 #define X__Y -#undef for +#undef for // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #undef final #undef __HAVE_X #undef _HAVE_X @@ -22,30 +22,30 @@ #define extern #define inline -#undef while -#undef const -#undef static -#undef extern -#undef inline +#undef while // expected-warning {{keyword or identifier with special meaning is used as a macro name}} +#undef const // expected-warning {{keyword or identifier with special meaning is used as a macro name}} +#undef static // expected-warning {{keyword or identifier with special meaning is used as a macro name}} +#undef extern // expected-warning {{keyword or identifier with special meaning is used as a macro name}} +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define inline __inline -#undef inline +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define inline __inline__ -#undef inline +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define inline inline__ // expected-warning {{keyword is hidden by macro definition}} -#undef inline +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define extern __inline // expected-warning {{keyword is hidden by macro definition}} -#undef extern +#undef extern // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define extern __extern // expected-warning {{keyword is hidden by macro definition}} -#undef extern +#undef extern // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define extern __extern__ // expected-warning {{keyword is hidden by macro definition}} -#undef extern +#undef extern // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define inline _inline // expected-warning {{keyword is hidden by macro definition}} -#undef inline +#undef inline // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #define volatile // expected-warning {{keyword is hidden by macro definition}} -#undef volatile +#undef volatile // expected-warning {{keyword or identifier with special meaning is used as a macro name}} #pragma clang diagnostic warning "-Wreserved-macro-identifier" diff --git a/clang/test/Preprocessor/warn-macro-undef-true.c b/clang/test/Preprocessor/warn-macro-undef-true.c index 9a64d577a96ed..e87b2d34f59cb 100644 --- a/clang/test/Preprocessor/warn-macro-undef-true.c +++ b/clang/test/Preprocessor/warn-macro-undef-true.c @@ -2,11 +2,7 @@ // RUN: %clang_cc1 %s -Eonly -std=c99 -verify=undef-true // RUN: %clang_cc1 %s -Eonly -std=c11 -verify=undef-true // RUN: %clang_cc1 %s -Eonly -std=c17 -verify=undef-true -// RUN: %clang_cc1 %s -Eonly -std=c23 -verify=undef-true - -#if __STDC_VERSION__ >= 202311L -/* undef-true-no-diagnostics */ -#endif +// RUN: %clang_cc1 %s -Eonly -std=c23 -verify=undef-true,c23-keyword #define FOO true #if FOO /* #1 */ @@ -27,7 +23,7 @@ /* undef-true-warning@#3 {{'true' is not defined, evaluates to 0}} */ #endif -#define true 1 +#define true 1 /* c23-keyword-warning {{keyword is hidden by macro definition}} */ #define FOO true #if FOO diff --git a/clang/test/Sema/c++-keyword-in-c.c b/clang/test/Sema/c++-keyword-in-c.c index f417469441114..89b5edbabbd9c 100644 --- a/clang/test/Sema/c++-keyword-in-c.c +++ b/clang/test/Sema/c++-keyword-in-c.c @@ -200,10 +200,10 @@ int import; int override; int final; -// We do not diagnose use of C++ keywords when used as a macro name because -// that does not conflict with C++ (the macros will be replaced before the -// keyword is seen by the parser). -#define this 12 +// In C mode, we do not diagnose use of C++ keywords when used as a macro name +// because that does not conflict with C++ (the macros will be replaced before +// the keyword is seen by the parser). +#define this 12 // cxx-warning {{keyword is hidden by macro definition}} // FIXME: These tests are disabled for C++ because it causes a crash. // See GH114815. diff --git a/clang/test/SemaCUDA/noinline.cu b/clang/test/SemaCUDA/noinline.cu index ce4dfe35ceaa8..e8e45c127eda2 100644 --- a/clang/test/SemaCUDA/noinline.cu +++ b/clang/test/SemaCUDA/noinline.cu @@ -13,7 +13,7 @@ __attribute__((__noinline__)) void fun3() { } #define __noinline__ __attribute__((__noinline__)) __noinline__ void fun5() {} -#undef __noinline__ +#undef __noinline__ // pedantic-warning {{keyword or identifier with special meaning is used as a macro name}} #10 "cuda.h" 3 // pedantic-warning {{this style of line directive is a GNU extension}} #define __noinline__ __attribute__((__noinline__)) __noinline__ void fun6() {} diff --git a/clang/test/SemaCXX/abstract.cpp b/clang/test/SemaCXX/abstract.cpp index 730d7e390f1d0..70c972261853c 100644 --- a/clang/test/SemaCXX/abstract.cpp +++ b/clang/test/SemaCXX/abstract.cpp @@ -4,6 +4,7 @@ #define __CONCAT(__X, __Y) __CONCAT1(__X, __Y) #define __CONCAT1(__X, __Y) __X ## __Y +// expected-warning@+1 {{keyword is hidden by macro definition}} #define static_assert(__b, __m) \ typedef int __CONCAT(__sa, __LINE__)[__b ? 1 : -1] #endif diff --git a/clang/test/SemaCXX/consteval-builtin.cpp b/clang/test/SemaCXX/consteval-builtin.cpp index 3ba95b4dbd9b5..2bb0d43ac3557 100644 --- a/clang/test/SemaCXX/consteval-builtin.cpp +++ b/clang/test/SemaCXX/consteval-builtin.cpp @@ -26,7 +26,7 @@ // c-error@-4 {{does not have the constexpr builtin}} #if __cplusplus < 201103L -#define static_assert __extension__ _Static_assert +#define static_assert __extension__ _Static_assert // c-warning {{keyword is hidden by macro definition}} #define CONSTEXPR11 #else #define CONSTEXPR11 constexpr diff --git a/clang/test/SemaTemplate/instantiate-array.cpp b/clang/test/SemaTemplate/instantiate-array.cpp index 41d1cfe138ab5..f317be4d22bac 100644 --- a/clang/test/SemaTemplate/instantiate-array.cpp +++ b/clang/test/SemaTemplate/instantiate-array.cpp @@ -1,10 +1,10 @@ // RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11 -// expected-no-diagnostics #ifndef __GXX_EXPERIMENTAL_CXX0X__ #define __CONCAT(__X, __Y) __CONCAT1(__X, __Y) #define __CONCAT1(__X, __Y) __X ## __Y +// expected-warning@+1 {{keyword is hidden by macro definition}} #define static_assert(__b, __m) \ typedef int __CONCAT(__sa, __LINE__)[__b ? 1 : -1] #endif diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html index 16ebeb1052b79..dfe2172ab2590 100755 --- a/clang/www/cxx_status.html +++ b/clang/www/cxx_status.html @@ -448,7 +448,19 @@ <h2 id="cxx26">C++2c implementation status</h2> <tr> <td>Preprocessing is never undefined</td> <td><a href="https://wg21.link/P2843";>P2843R3</a></td> - <td class="none" align="center">No</td> + <td class="partial" align="center"> + <details> + <summary>Clang 24 (Partial)</summary> + Error-by-default warning for #define keyword; + Pedantic warning for #undef keyword; + #line is covered by P4136R2; + For attributes, there has been no discussions - The conservative approach is to do nothing; + For define in macros, the discussion doesn't seem to have reached its course yet. + + The changes in this paper may break some legacy code, so we have tried to minimize the breakage + while providing users with clear diagnostic information to avoid introducing new undefined behaviors. + </details> + </td> </tr> 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