[clang] [clang][Sema] Crash due to asm labeled global vars of undefined type (PR #219746)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt created https://github.com/llvm/llvm-project/pull/219746

CheckAsmLabel fails to check for an incomplete type before attempting to get the type layout. Short circuit on an incomplete type as Sema will already reject a global with an incomplete type.

>From 5a494bca90d98bc51accc6bf8a5d7fb9c63f676a Mon Sep 17 00:00:00 2001
From: Oliver Hunt <oliver at apple.com>
Date: Sat, 29 Aug 2026 00:14:45 -0600
Subject: [PATCH] [clang][Sema] Crash due to asm labeled global vars of
 undefined type

CheckAsmLabel fails to check for an incomplete type before attempting
to get the type layout. Short circuit on an incomplete type as Sema
will already reject a global with an incomplete type.
---
 clang/lib/Sema/SemaDecl.cpp                              | 4 ++++
 .../test/Sema/global-explicit-register-undefined-type.c  | 9 +++++++++
 2 files changed, 13 insertions(+)
 create mode 100644 clang/test/Sema/global-explicit-register-undefined-type.c

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 07c6157ab8f31..4850cb5a6cb9f 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -7756,6 +7756,10 @@ void Sema::CheckAsmLabel(Scope *S, Expr *E, StorageClass SC,
   StringLiteral *SE = cast<StringLiteral>(E);
   StringRef Label = SE->getString();
   QualType R = TInfo->getType();
+  if (RequireCompleteType(NewVD->getBeginLoc(), R, diag::err_typecheck_decl_incomplete_type)) {
+    NewVD->setInvalidDecl();
+    return;
+  }
   if (S->getFnParent() != nullptr) {
     switch (SC) {
     case SC_None:
diff --git a/clang/test/Sema/global-explicit-register-undefined-type.c b/clang/test/Sema/global-explicit-register-undefined-type.c
new file mode 100644
index 0000000000000..690dc0a9efddb
--- /dev/null
+++ b/clang/test/Sema/global-explicit-register-undefined-type.c
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 %s -triple arm64-apple-macosx -fsyntax-only -verify
+
+register struct Undefined1 bar asm("x1"); // #inline-type-def
+// expected-error@#inline-type-def {{variable has incomplete type 'struct Undefined1'}}
+// expected-note@#inline-type-def {{forward declaration of 'struct Undefined1'}}
+struct Undefined2; // #outline-type-def
+register struct Undefined2 bar asm("x1"); // #outline-type-label
+// expected-error@#outline-type-label {{variable has incomplete type 'struct Undefined2'}}
+// expected-note@#outline-type-def {{forward declaration of 'struct Undefined2'}}