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

>From 5aec6a44a91e416b39856cf2e1d04f2cdfa1f1ff Mon Sep 17 00:00:00 2001
From: Oliver Hunt <[email protected]>
Date: Sat, 29 Aug 2026 00:14:45 -0600
Subject: [PATCH] [clang][Sema] Crash due to asm labeled incomplete global
 register decls

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'}}

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