[clang] [C] static_assert in a for loop is not an extension (PR #151955)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman created https://github.com/llvm/llvm-project/pull/151955

The original wording can be squinted at to pretend this was always allowed. GCC squints at it that way, so we're doing the same and no longer issuing an extension diagnostic for use of static_assert in the condition-1 of a for loop in C.

Fixes #149633

>From 8da6154930f3fc4f9fa20730e7f93e0fc5bc317d Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Mon, 4 Aug 2025 08:49:18 -0400
Subject: [PATCH] [C] static_assert in a for loop is not an extension

The original wording can be squinted at to pretend this was always
allowed. GCC squints at it that way, so we're doing the same and no
longer issuing an extension diagnostic for use of static_assert in the
condition-1 of a for loop in C.

Fixes #149633
---
 clang/lib/Sema/SemaStmt.cpp | 6 +++++-
 clang/test/Sema/for.c       | 3 +--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 3f89843a9081a..a5f92020f49f8 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -2284,7 +2284,11 @@ StmtResult Sema::ActOnForStmt(SourceLocation ForLoc, SourceLocation LParenLoc,
           // we can diagnose if we don't see any variable declarations. This
           // covers a case like declaring a typedef, function, or structure
           // type rather than a variable.
-          NonVarSeen = DI;
+          //
+          // Note, _Static_assert is acceptable because it does not declare an
+          // identifier at all, so "for object having" does not apply.
+          if (!isa<StaticAssertDecl>(DI))
+            NonVarSeen = DI;
         }
       }
       // Diagnose if we saw a non-variable declaration but no variable
diff --git a/clang/test/Sema/for.c b/clang/test/Sema/for.c
index e16169aac0c4c..35c4720ef3305 100644
--- a/clang/test/Sema/for.c
+++ b/clang/test/Sema/for.c
@@ -26,6 +26,5 @@ void b11 (void) { for (static _Thread_local struct { int i; } s;s.i;); } /* c11-
 #endif
 
 void b12(void) {
-  for(_Static_assert(1, "");;) {} /* c11-warning {{non-variable declaration in 'for' loop is a C23 extension}}
-                                     c23-warning {{non-variable declaration in 'for' loop is incompatible with C standards before C23}} */
+  for(_Static_assert(1, "");;) {} /* okay, _Static_assert declares *no* identifiers */
 }