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[C] Fix a false-positive with tentative defn compat #139738

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Merged
merged 1 commit into from
May 13, 2025
Merged

[C] Fix a false-positive with tentative defn compat #139738

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12 changes: 10 additions & 2 deletions 12 clang/lib/Sema/SemaDecl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4755,8 +4755,16 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
return;
}
} else {
Diag(New->getLocation(), diag::warn_cxx_compat_tentative_definition) << New;
Diag(Old->getLocation(), diag::note_previous_declaration);
// C++ may not have a tentative definition rule, but it has a different
// rule about what constitutes a definition in the first place. See
// [basic.def]p2 for details, but the basic idea is: if the old declaration
// contains the extern specifier and doesn't have an initializer, it's fine
// in C++.
if (Old->getStorageClass() != SC_Extern || Old->hasInit()) {
Diag(New->getLocation(), diag::warn_cxx_compat_tentative_definition)
<< New;
Diag(Old->getLocation(), diag::note_previous_declaration);
}
}

if (haveIncompatibleLanguageLinkages(Old, New)) {
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5 changes: 4 additions & 1 deletion 5 clang/test/Sema/warn-tentative-defn-compat.c
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Expand Up @@ -20,4 +20,7 @@ int k = 12; // expected-warning {{duplicate declaration of 'k' is invalid in C++
cxx-error {{redefinition of 'k'}}

// Cannot have two declarations with initializers, that is a redefinition in
// both C and C++.
// both C and C++. However, C++ does have a different definition of what makes
// a declaration a definition.
extern const int a;
const int a = 12; // Okay in C and C++
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