[clang] 9e36e5a - [C++20] [Modules] Don't check redeclaration for TagUseKind::Referencekind declaration (#194546)

[via cfe-commits]

Author: Chuanqi Xu
Date: 2026-09-21T09:41:31Z
New Revision: 9e36e5a44bb5ade74202b01246cab4231d202972

URL: https://github.com/llvm/llvm-project/commit/9e36e5a44bb5ade74202b01246cab4231d202972
DIFF: https://github.com/llvm/llvm-project/commit/9e36e5a44bb5ade74202b01246cab4231d202972.diff

LOG: [C++20] [Modules] Don't check redeclaration for TagUseKind::Referencekind declaration (#194546)

Close https://github.com/llvm/llvm-project/issues/72038

The reason of the issue is ISO forbids redeclaration between GMF and the
module purview.

But "struct kevent evt;" was thought to be declaration than triggers the
above issue.

In this patch, we simply not checking for cases of TagUseKind::Reference
declaration.

Added: 
    clang/test/Modules/pr72038.cppm

Modified: 
    clang/lib/Sema/SemaDecl.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index e52f99b4cd98d..db5e66cb96c3e 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -19189,7 +19189,12 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
   if (!Invalid && SearchDC->isRecord())
     SetMemberAccessSpecifier(New, PrevDecl, AS);
 
-  if (PrevDecl)
+  // FIXME: An elaborated-type-specifier referring to an existing tag should
+  // ideally not introduce a redeclaration. ActOnTag currently creates one, so
+  // avoid diagnosing it as a redeclaration across module boundaries.
+  //
+  // See https://github.com/llvm/llvm-project/pull/194546 for full background.
+  if (PrevDecl && TUK != TagUseKind::Reference)
     CheckRedeclarationInModule(New, PrevDecl);
 
   if (TUK == TagUseKind::Definition) {

diff  --git a/clang/test/Modules/pr72038.cppm b/clang/test/Modules/pr72038.cppm
new file mode 100644
index 0000000000000..bd1cdc002b0ff
--- /dev/null
+++ b/clang/test/Modules/pr72038.cppm
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -std=c++20 %s -fsyntax-only -verify
+
+// expected-no-diagnostics
+module;
+struct kevent { };
+void kevent(int x);
+export module my_mod;
+struct kevent evt;