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[clang] fix obtaining underlying type for demoted enum definitions #155900

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Merged
merged 1 commit into from
Aug 28, 2025
Merged

[clang] fix obtaining underlying type for demoted enum definitions #155900

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10 changes: 8 additions & 2 deletions clang/lib/Sema/SemaDecl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -18544,8 +18544,14 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
if (PrevDecl)
CheckRedeclarationInModule(New, PrevDecl);

if (TUK == TagUseKind::Definition && (!SkipBody || !SkipBody->ShouldSkip))
New->startDefinition();
if (TUK == TagUseKind::Definition) {
if (!SkipBody || !SkipBody->ShouldSkip) {
New->startDefinition();
} else {
New->setCompleteDefinition();
New->demoteThisDefinitionToDeclaration();
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Do we ever come here for non-enum tag types? I looked up the ShouldSkipInfo class, but I wasn't able to figure out exactly when or why we skip tag bodies.

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The case we were hitting on this regression was from a recent improvement implemented here: 2db239a

But this skipping of parsing of bodies is built upon much older patch: d9ba224

But yes, we should come here for other tag kinds as well, based on the tests added in the first commit.

}
}

ProcessDeclAttributeList(S, New, Attrs);
AddPragmaAttributes(S, New);
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9 changes: 8 additions & 1 deletion clang/lib/Sema/SemaType.cpp
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Expand Up @@ -9878,7 +9878,14 @@ static QualType GetEnumUnderlyingType(Sema &S, QualType BaseType,
S.DiagnoseUseOfDecl(ED, Loc);

QualType Underlying = ED->getIntegerType();
assert(!Underlying.isNull());
if (Underlying.isNull()) {
// This is an enum without a fixed underlying type which we skipped parsing
// the body because we saw its definition previously in another module.
// Use the definition's integer type in that case.
assert(ED->isThisDeclarationADemotedDefinition());
Underlying = ED->getDefinition()->getIntegerType();
assert(!Underlying.isNull());
}

return Underlying;
}
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17 changes: 17 additions & 0 deletions clang/test/Modules/GH155028-1.cpp
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@@ -0,0 +1,17 @@
// RUN: %clang_cc1 -std=c++20 -verify %s
// expected-no-diagnostics

#pragma clang module build M
module "M" {
module "A" {}
module "B" {}
}
#pragma clang module contents
#pragma clang module begin M.A
enum E1 {};
#pragma clang module end
#pragma clang module begin M.B
enum E1 {};
using T = __underlying_type(E1);
#pragma clang module end
#pragma clang module endbuild