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[clang] [modules] Implement P3618R0: Allow attaching main to the global module #146461

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Merged
merged 7 commits into from
Jul 2, 2025
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[clang] [modules] Implement P3618R0: Allow attaching main to the global module #146461

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845f998
Implement P3618R0
kish1n Jul 1, 2025
70afaf3
typo
kish1n Jul 1, 2025
880f14b
clang-format
kish1n Jul 1, 2025
fd91a72
fix libcxx test
kish1n Jul 1, 2025
035592a
darker format
kish1n Jul 1, 2025
bf5dd70
fix p3 test
kish1n Jul 1, 2025
6ebbad4
update cxx_status.html
kish1n Jul 1, 2025
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2 changes: 2 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -135,6 +135,8 @@ C++2c Feature Support

- Implemented `P2719R4 Type-aware allocation and deallocation functions <https://wg21.link/P2719>`_.

- Implemented `P3618R0 Allow attaching main to the global module <https://wg21.link/P3618>`_.

C++23 Feature Support
^^^^^^^^^^^^^^^^^^^^^

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6 changes: 3 additions & 3 deletions clang/include/clang/Basic/DiagnosticSemaKinds.td
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Expand Up @@ -1084,9 +1084,9 @@ def warn_main_redefined : Warning<"variable named 'main' with external linkage "
"has undefined behavior">, InGroup<Main>;
def ext_main_used : Extension<
"referring to 'main' within an expression is a Clang extension">, InGroup<Main>;
def ext_main_invalid_linkage_specification : ExtWarn<
"'main' should not be "
"'extern \"%select{C|C++}0\"'">, InGroup<Main>;
def ext_main_invalid_linkage_specification : ExtWarn<"'main' should not be "
"'extern \"C\"'">,
InGroup<Main>;

/// parser diagnostics
def ext_no_declarators : ExtWarn<"declaration does not declare anything">,
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9 changes: 3 additions & 6 deletions clang/lib/Sema/SemaDecl.cpp
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Expand Up @@ -12400,12 +12400,9 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,

void Sema::CheckMain(FunctionDecl *FD, const DeclSpec &DS) {
// [basic.start.main]p3
// The main function shall not be declared with a linkage-specification.
if (FD->isExternCContext() ||
(FD->isExternCXXContext() &&
FD->getDeclContext()->getRedeclContext()->isTranslationUnit()))
Diag(FD->getLocation(), diag::ext_main_invalid_linkage_specification)
<< FD->getLanguageLinkage();
// The main function shall not be declared with C linkage-specification.
if (FD->isExternCContext())
Diag(FD->getLocation(), diag::ext_main_invalid_linkage_specification);
Comment on lines +12403 to +12405
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@ChuanqiXu9 ChuanqiXu9 Jul 1, 2025

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I feel this is not right. We should avoid emitting the diagnostics if it is in a named module.

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@kish1n kish1n Jul 1, 2025

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According to proposal, extern "C" for main is still ill-formed regardless if it is named module or not. On other hand, extern "C++" is allowed whether in named module or not.


// C++11 [basic.start.main]p3:
// A program that [...] declares main to be inline, static or
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5 changes: 3 additions & 2 deletions clang/test/CXX/basic/basic.start/basic.start.main/p3.cpp
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Expand Up @@ -102,11 +102,12 @@ namespace ns {
}

#elif TEST13
// expected-no-diagnostics
extern "C++" {
int main(); // expected-warning {{'main' should not be 'extern "C++"'}}
int main();
}

extern "C++" int main(); // expected-warning {{'main' should not be 'extern "C++"'}}
extern "C++" int main();

namespace ns1 {
extern "C++" int main(); // ok
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2 changes: 2 additions & 0 deletions clang/test/SemaCXX/modules.cppm
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Expand Up @@ -68,6 +68,8 @@ int n;
//--- test3.cpp
export module bar;

extern "C++" int main() {}

static int m;

int n;
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2 changes: 1 addition & 1 deletion clang/www/cxx_status.html
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Expand Up @@ -317,7 +317,7 @@ <h2 id="cxx26">C++2c implementation status</h2>
<tr>
<td>Attaching main to the global module</td>
<td><a href="https://wg21.link/P3618">P3618R0</a> (<a href="#dr">DR</a>)</td>
<td class="none" align="center">No</td>
<td class="unreleased" align="center">Clang 21</td>
</tr>
<tr>
<td>Expansion Statements</td>
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16 changes: 13 additions & 3 deletions libcxx/utils/libcxx/test/features.py
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Expand Up @@ -336,12 +336,22 @@ def _mingwSupportsModules(cfg):
or platform.system().lower().startswith("aix")
# Avoid building on platforms that don't support modules properly.
or not hasCompileFlag(cfg, "-Wno-reserved-module-identifier")
or not sourceBuilds(
cfg,
"""
# older versions don't support extern "C++", newer versions don't support main in named module.
or not (
sourceBuilds(
cfg,
"""
export module test;
extern "C++" int main(int, char**) { return 0; }
""",
)
or sourceBuilds(
cfg,
"""
export module test;
int main(int, char**) { return 0; }
""",
)
),
),
# The time zone validation tests compare the output of zdump against the
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