[PATCH] D115094: Fix -Wdeclaration-after-statement doesn't work when used with -std=c99

ram via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Sat Dec 4 01:41:01 PST 2021

ram7bhupal created this revision.
ram7bhupal added reviewers: dblaikie, aaron.ballman.
ram7bhupal requested review of this revision.
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-Wdeclaration-after-statement doesn't do anything if combined with -std=c99 or newer.

Take a look at the following program:

// prog.c
#include <stdio.h>

int main(void)

  printf("hello world\n");
  int i = 0;
  return 0;


If I compile it with clang with the following command:

$ clang -std=c99 -Wdeclaration-after-statement prog.c

it produces no warnings.

If I compile the same code with gcc with the following command:

$ gcc -std=c99 -Wdeclaration-after-statement prog.c

it produces the following warning:

prog.c: In function ‘main’:
prog.c:6:9: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]

  6 |         int i = 0;
    |         ^~~

This is the behavior I would like to have with clang, but it only produces this warning if I use it with -std=c90 or -std=c89 or -ansi, like this:

$ clang -std=c90 -Wdeclaration-after-statement prog.c
prog.c:6:6: warning: ISO C90 forbids mixing declarations and code [-Wdeclaration-after-statement]

  int i = 0;

1 warning generated.

  rG LLVM Github Monorepo



Index: clang/lib/Sema/SemaStmt.cpp
--- clang/lib/Sema/SemaStmt.cpp
+++ clang/lib/Sema/SemaStmt.cpp
@@ -410,9 +410,9 @@
                                    ArrayRef<Stmt *> Elts, bool isStmtExpr) {
   const unsigned NumElts = Elts.size();
-  // If we're in C89 mode, check that we don't have any decls after stmts.  If
+  // Check that we don't have any decls after stmts.  If
   // so, emit an extension diagnostic.
-  if (!getLangOpts().C99 && !getLangOpts().CPlusPlus) {
+  if (!getLangOpts().CPlusPlus) {
     // Note that __extension__ can be around a decl.
     unsigned i = 0;
     // Skip over all declarations.

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