release/21.x: [C] static_assert in a for loop is not an extension (#151955)

[llvmbot]

Backport cb50d78

Requested by: @AaronBallman

[llvmbot]

@cor3ntin What do you think about merging this PR to the release branch?

[llvmbot]

@llvm/pr-subscribers-clang

Author: None (llvmbot)

Changes

Backport cb50d78

Requested by: @AaronBallman

---

Full diff: https://github.com/llvm/llvm-project/pull/151999.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaStmt.cpp (+5-1)
• (modified) clang/test/Sema/for.c (+1-2)

diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index f85826aecadf3..f46be75bda20f 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -2287,7 +2287,11 @@ 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 */
 }

[github-actions]

@AaronBallman (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.