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Allow weak/selectany external definitions in header units. #162713

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Merged
merged 1 commit into from
Oct 16, 2025
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Allow weak/selectany external definitions in header units. #162713

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27 changes: 21 additions & 6 deletions clang/lib/Sema/SemaDecl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -13816,13 +13816,20 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
VDecl->setInvalidDecl();
}

// C++ [module.import/6] external definitions are not permitted in header
// units.
// C++ [module.import/6]
// ...
// A header unit shall not contain a definition of a non-inline function or
// variable whose name has external linkage.
//
// We choose to allow weak & selectany definitions, as they are common in
// headers, and have semantics similar to inline definitions which are allowed
// in header units.
if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
!VDecl->isInvalidDecl() && VDecl->isThisDeclarationADefinition() &&
VDecl->getFormalLinkage() == Linkage::External && !VDecl->isInline() &&
!VDecl->isTemplated() && !isa<VarTemplateSpecializationDecl>(VDecl) &&
!VDecl->getInstantiatedFromStaticDataMember()) {
!VDecl->getInstantiatedFromStaticDataMember() &&
!(VDecl->hasAttr<SelectAnyAttr>() || VDecl->hasAttr<WeakAttr>())) {
Diag(VDecl->getLocation(), diag::err_extern_def_in_header_unit);
VDecl->setInvalidDecl();
}
Expand Down Expand Up @@ -16153,16 +16160,24 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
}
}

// C++ [module.import/6] external definitions are not permitted in header
// units. Deleted and Defaulted functions are implicitly inline (but the
// C++ [module.import/6]
// ...
// A header unit shall not contain a definition of a non-inline function or
// variable whose name has external linkage.
//
// Deleted and Defaulted functions are implicitly inline (but the
// inline state is not set at this point, so check the BodyKind explicitly).
// We choose to allow weak & selectany definitions, as they are common in
// headers, and have semantics similar to inline definitions which are allowed
// in header units.
// FIXME: Consider an alternate location for the test where the inlined()
// state is complete.
if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
!FD->isInvalidDecl() && !FD->isInlined() &&
BodyKind != FnBodyKind::Delete && BodyKind != FnBodyKind::Default &&
FD->getFormalLinkage() == Linkage::External && !FD->isTemplated() &&
!FD->isTemplateInstantiation()) {
!FD->isTemplateInstantiation() &&
!(FD->hasAttr<SelectAnyAttr>() || FD->hasAttr<WeakAttr>())) {
assert(FD->isThisDeclarationADefinition());
Diag(FD->getLocation(), diag::err_extern_def_in_header_unit);
FD->setInvalidDecl();
Expand Down
13 changes: 13 additions & 0 deletions clang/test/CXX/module/module.import/p6.cpp
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Expand Up @@ -3,6 +3,9 @@

// RUN: %clang_cc1 -std=c++20 -x c++-header %t/bad-header-unit.h \
// RUN: -emit-header-unit -o %t/bad-header-unit.pcm -verify
// RUN: %clang_cc1 -std=c++20 -x c++-header %t/bad-header-unit-declspec.h \
// RUN: -emit-header-unit -o %t/bad-header-unit.pcm -verify \
// RUN: -fdeclspec

//--- bad-header-unit.h

Expand Down Expand Up @@ -77,3 +80,13 @@ template <typename T> bool b() {
}

inline bool B = b<int>();

__attribute__((weak)) int weak_fun_definition() { return 42; }

__attribute__((weak)) int weak_var_definition = 42;

//--- bad-header-unit-declspec.h

/* The cases below should compile without diagnostics. */

__declspec(selectany) int selectany_var_definition = 42; // expected-no-diagnostics
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