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[C23] Implement N2490, Remove trigraphs??!
This follows the same implementation logic as with C++ and is compatible with the GCC behavior in C. Trigraphs are enabled by default in -std=c* conformance modes before C23, but are disabled in GNU and Microsoft modes as well as in C23 or later.
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‎clang/docs/ReleaseNotes.rst

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@@ -271,6 +271,13 @@ C23 Feature Support
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previously implemented allowing a label at the end of a compound statement,
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and now we've implemented allowing a label to be followed by a declaration
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instead of a statement.
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- Implemented
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`N2940 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2940.pdf>`_ which
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removes support for trigraphs in C23 and later. In earlier language modes,
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trigraphs remain enabled by default in conforming modes (e.g. ``-std=c17``)
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and disabled by default in GNU and Microsoft modes (e.g., ``-std=gnu17`` or
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``-fms-compatibility``). If needed, you can enable trigraphs by passing
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``-ftrigraphs``.
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Non-comprehensive list of changes in this release
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-------------------------------------------------

‎clang/lib/Frontend/CompilerInvocation.cpp

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@@ -3480,7 +3480,8 @@ void CompilerInvocationBase::GenerateLangArgs(const LangOptions &Opts,
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Twine(Major) + "." + Twine(Minor) + "." + Twine(Subminor));
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}
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if ((!Opts.GNUMode && !Opts.MSVCCompat && !Opts.CPlusPlus17) || T.isOSzOS()) {
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if ((!Opts.GNUMode && !Opts.MSVCCompat && !Opts.CPlusPlus17 && !Opts.C23) ||
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T.isOSzOS()) {
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if (!Opts.Trigraphs)
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GenerateArg(Consumer, OPT_fno_trigraphs);
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} else {
@@ -3876,10 +3877,11 @@ bool CompilerInvocation::ParseLangArgs(LangOptions &Opts, ArgList &Args,
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// Mimicking gcc's behavior, trigraphs are only enabled if -trigraphs
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// is specified, or -std is set to a conforming mode.
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// Trigraphs are disabled by default in c++1z onwards.
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// Trigraphs are disabled by default in C++17 and C23 onwards.
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// For z/OS, trigraphs are enabled by default (without regard to the above).
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Opts.Trigraphs =
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(!Opts.GNUMode && !Opts.MSVCCompat && !Opts.CPlusPlus17) || T.isOSzOS();
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(!Opts.GNUMode && !Opts.MSVCCompat && !Opts.CPlusPlus17 && !Opts.C23) ||
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T.isOSzOS();
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Opts.Trigraphs =
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Args.hasFlag(OPT_ftrigraphs, OPT_fno_trigraphs, Opts.Trigraphs);
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‎clang/test/C/C2x/n2940.c

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// RUN: %clang_cc1 -verify=no-trigraphs -std=c23 %s
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// RUN: %clang_cc1 -verify=no-trigraphs -std=gnu23 %s
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// RUN: %clang_cc1 -verify=no-trigraphs -std=gnu17 %s
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// RUN: %clang_cc1 -verify=no-trigraphs -std=gnu11 %s
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// RUN: %clang_cc1 -verify=no-trigraphs -std=gnu99 %s
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// RUN: %clang_cc1 -verify=no-trigraphs -std=gnu89 %s
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// RUN: %clang_cc1 -verify=trigraphs -std=c17 %s
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// RUN: %clang_cc1 -verify=trigraphs -std=c11 %s
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// RUN: %clang_cc1 -verify=trigraphs -std=c99 %s
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// RUN: %clang_cc1 -verify=trigraphs -std=c89 %s
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// RUN: %clang_cc1 -verify=trigraphs -std=c23 -ftrigraphs %s
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/* WG14 N2940: Clang 18
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* Removing trigraphs??!
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*/
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// Trigraphs are enabled by default in any conforming C mode before C23, but
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// are otherwise disabled (in all GNU modes, and in C23 or later).
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// The ??= trigraph, if supported, will become the # character, which is a null
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// preprocessor directive that does nothing.
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??=
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// no-trigraphs-warning@-1 {{trigraph ignored}} \
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no-trigraphs-error@-1 {{expected identifier or '('}} \
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trigraphs-warning@-1 {{trigraph converted to '#' character}}
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‎clang/test/C/drs/dr3xx.c

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/* RUN: %clang_cc1 -std=c89 -fsyntax-only -Wvla -verify=expected,c89only -pedantic -Wno-c11-extensions %s
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RUN: %clang_cc1 -std=c99 -fsyntax-only -Wvla -verify=expected,c99andup -pedantic -Wno-c11-extensions %s
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RUN: %clang_cc1 -std=c11 -fsyntax-only -Wvla -verify=expected,c99andup -pedantic %s
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RUN: %clang_cc1 -std=c17 -fsyntax-only -Wvla -verify=expected,c99andup -pedantic %s
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RUN: %clang_cc1 -std=c2x -fsyntax-only -Wvla -verify=expected,c99andup -pedantic %s
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/* RUN: %clang_cc1 -std=c89 -fsyntax-only -Wvla -verify=expected,c89only,c17andearlier -pedantic -Wno-c11-extensions %s
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RUN: %clang_cc1 -std=c99 -fsyntax-only -Wvla -verify=expected,c99andup,c17andearlier -pedantic -Wno-c11-extensions %s
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RUN: %clang_cc1 -std=c11 -fsyntax-only -Wvla -verify=expected,c99andup,c17andearlier -pedantic %s
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RUN: %clang_cc1 -std=c17 -fsyntax-only -Wvla -verify=expected,c99andup,c17andearlier -pedantic %s
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RUN: %clang_cc1 -std=c2x -fsyntax-only -Wvla -verify=expected,c99andup,c23andup -pedantic %s
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*/
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/* The following are DRs which do not require tests to demonstrate
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#error "We definitely should not have gotten here"
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#endif
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/* WG14 DR309: yes
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* Clarifying trigraph substitution
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*/
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int dr309??(1??) = { 1 }; /* expected-warning {{trigraph converted to '[' character}}
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expected-warning {{trigraph converted to ']' character}}
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*/
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/* WG14 DR311: yes
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* Definition of variably modified types
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*/
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char (**a)[10 * sizeof f]) {
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_Static_assert(sizeof **a == sizeof(long double) * 10, "");
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}
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/* WG14 DR309: yes
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* Clarifying trigraph substitution
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*/
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int dr309??(1??) = { 1 }; /* c17andearlier-warning {{trigraph converted to '[' character}}
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c17andearlier-warning {{trigraph converted to ']' character}}
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c23andup-warning 2 {{trigraph ignored}}
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c23andup-error {{expected ';' after top level declarator}}
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*/
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/* NOTE: Due to interactions with the diagnostic system, dr309 should be the
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* last test case in this file because subsequent diagnostics may not be
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* generated as expected.
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*/

‎clang/test/Preprocessor/ucn-pp-identifier.c

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// RUN: %clang_cc1 %s -fsyntax-only -std=c99 -pedantic -verify=expected,ext -Wundef -DTRIGRAPHS=1
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// RUN: %clang_cc1 %s -fsyntax-only -std=c2x -pedantic -verify=expected,ext -Wundef -DTRIGRAPHS=1
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// RUN: %clang_cc1 %s -fsyntax-only -std=c23 -pedantic -verify=expected,ext -Wundef -ftrigraphs -DTRIGRAPHS=1
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// RUN: %clang_cc1 %s -fsyntax-only -x c++ -pedantic -verify=expected,ext -Wundef -fno-trigraphs
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// RUN: %clang_cc1 %s -fsyntax-only -x c++ -std=c++23 -pedantic -ftrigraphs -DTRIGRAPHS=1 -verify=expected,cxx23 -Wundef -Wpre-c++23-compat
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// RUN: %clang_cc1 %s -fsyntax-only -x c++ -pedantic -verify=expected,ext -Wundef -ftrigraphs -DTRIGRAPHS=1

‎clang/www/c_status.html

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@@ -1156,7 +1156,7 @@ <h2 id="c2x">C23 implementation status</h2>
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<tr>
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<td>Remove trigraphs??!</td>
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<td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2940.pdf">N2940</a></td>
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<td class="full" align="center">Yes</td>
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<td class="unreleased" align="center">Clang 18</td>
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</tr>
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<tr>
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<td>Improved normal enumerations</td>

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