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

Author: mizvekov

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);

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;
 }

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