[clang] [clang][Sema] Fix crash when applying sentinel to block variable (PR #173866)

[via cfe-commits]

https://github.com/nataliakokoromyti created https://github.com/llvm/llvm-project/pull/173866

This PR avoids the crash in Clang Sema when the sentinel attribute is applied to a block variable (see [issue](https://github.com/llvm/llvm-project/issues/173820))

basically, handleSentinelAttr always treated the type as a function with a prototype which is not true for block variables like void (^a)() and it caused Clang to crash. Now Clang will print a warning instead of crashing. 




>From 2f4aaf3c0082cb40a72750e1779492b5f07afdea Mon Sep 17 00:00:00 2001
From: Natalia Kokoromyti <nataliakokoromyti at gmail.com>
Date: Mon, 29 Dec 2025 04:26:15 -0800
Subject: [PATCH 1/2] add test for issue #173820

---
 clang/test/Sema/sentinel-attribute-block.c | 6 ++++++
 1 file changed, 6 insertions(+)
 create mode 100644 clang/test/Sema/sentinel-attribute-block.c

diff --git a/clang/test/Sema/sentinel-attribute-block.c b/clang/test/Sema/sentinel-attribute-block.c
new file mode 100644
index 0000000000000..65d3870f60e97
--- /dev/null
+++ b/clang/test/Sema/sentinel-attribute-block.c
@@ -0,0 +1,6 @@
+  // RUN: %clang_cc1 -fsyntax-only -fblocks -verify %s
+
+  // Test that sentinel attribute on block variables doesn't crash
+  void foo(void) {
+    void (^a)() __attribute__((__sentinel__)) = {}; // expected-warning {{'sentinel' attribute requires named arguments}}
+  }

>From 15ef1282c823d260bc5a3e46eeda31656ecdf7a7 Mon Sep 17 00:00:00 2001
From: Natalia Kokoromyti <nataliakokoromyti at gmail.com>
Date: Mon, 29 Dec 2025 04:40:03 -0800
Subject: [PATCH 2/2] fix issue #173820

---
 clang/lib/Sema/SemaDeclAttr.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 263ce2118ba86..774d8f56443dc 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -2979,6 +2979,10 @@ static void handleSentinelAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
                                    : Ty->castAs<BlockPointerType>()
                                          ->getPointeeType()
                                          ->castAs<FunctionType>();
+      if (isa<FunctionNoProtoType>(FT)) {
+        S.Diag(AL.getLoc(), diag::warn_attribute_sentinel_named_arguments);
+        return;
+      }
       if (!cast<FunctionProtoType>(FT)->isVariadic()) {
         int m = Ty->isFunctionPointerType() ? 0 : 1;
         S.Diag(AL.getLoc(), diag::warn_attribute_sentinel_not_variadic) << m;