From 2826b3afb0004a7b643e1ebe07a3fbd7eef4c93d Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov@gmail.com>
Date: Thu, 28 Aug 2025 14:59:47 -0300
Subject: [PATCH] [clang] fix obtaining underlying type for demoted enum
 definitions

Clang skips parsing a TagDecl definition in case a definition was already parsed
in another module.

In those cases, an EnumDecl might be left without an IntegerType.
Take this into account when getting the underlying type of an enum,
look for the integer type in the definition instead in those cases.

This patch also changes the implementation so it properly marks those
skipped tag definitions as demoted.

This fixes a regression reported here: https://github.com/llvm/llvm-project/pull/155028#issuecomment-3233410561

Since this regression was never released, there are no release notes.
---
 clang/lib/Sema/SemaDecl.cpp       | 10 ++++++++--
 clang/lib/Sema/SemaType.cpp       |  9 ++++++++-
 clang/test/Modules/GH155028-1.cpp | 17 +++++++++++++++++
 3 files changed, 33 insertions(+), 3 deletions(-)
 create mode 100644 clang/test/Modules/GH155028-1.cpp

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index a47c5ab3aaff7..73f92f7ec9a1e 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -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();
+    }
+  }
 
   ProcessDeclAttributeList(S, New, Attrs);
   AddPragmaAttributes(S, New);
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 3f31a05d382a6..0f655d7f684a5 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -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;
 }
diff --git a/clang/test/Modules/GH155028-1.cpp b/clang/test/Modules/GH155028-1.cpp
new file mode 100644
index 0000000000000..d60112b48c218
--- /dev/null
+++ b/clang/test/Modules/GH155028-1.cpp
@@ -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