[PATCH] D47840: Make -Wgcc-compat complain about declarations in for loop init statements

[George Burgess IV via Phabricator via cfe-commits]

george.burgess.iv created this revision.
george.burgess.iv added reviewers: rsmith, aaron.ballman.

The following code is invalid before C99, since we try to declare `i` inside the first clause of the for loop:

  void foo() {
    for (int i = 0; i < 10; i++);
  }

GCC does not accept this code in c89 or gnu89, but clang does: https://godbolt.org/g/ZWr3nA .

If the user cares about GCC compatibility, we should probably warn about this if we're not in C99.

I'm not 100% thrilled that we're emitting two warnings about the same thing for slightly different reasons; alternatives welcome. :)


Repository:
  rC Clang

https://reviews.llvm.org/D47840

Files:
  include/clang/Basic/DiagnosticParseKinds.td
  lib/Parse/ParseStmt.cpp
  test/Parser/gcc-for-loop-init-compatibility.c


Index: test/Parser/gcc-for-loop-init-compatibility.c
===================================================================
--- /dev/null
+++ test/Parser/gcc-for-loop-init-compatibility.c
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c89 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=gnu89 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c99 -fsyntax-only -verify %s -DC99
+
+#ifdef C99
+// expected-no-diagnostics
+#endif
+
+void foo() {
+#ifndef C99
+  // expected-warning at +2{{GCC does not allow variable declarations in for loop initializers before C99}}
+#endif
+  for (int i = 0; i < 10; i++)
+    ;
+}
Index: lib/Parse/ParseStmt.cpp
===================================================================
--- lib/Parse/ParseStmt.cpp
+++ lib/Parse/ParseStmt.cpp
@@ -1622,8 +1622,10 @@
     ForRange = true;
   } else if (isForInitDeclaration()) {  // for (int X = 4;
     // Parse declaration, which eats the ';'.
-    if (!C99orCXXorObjC)   // Use of C99-style for loops in C90 mode?
+    if (!C99orCXXorObjC) {   // Use of C99-style for loops in C90 mode?
       Diag(Tok, diag::ext_c99_variable_decl_in_for_loop);
+      Diag(Tok, diag::warn_gcc_variable_decl_in_for_loop);
+    }
 
     // In C++0x, "for (T NS:a" might not be a typo for ::
     bool MightBeForRangeStmt = getLangOpts().CPlusPlus;
Index: include/clang/Basic/DiagnosticParseKinds.td
===================================================================
--- include/clang/Basic/DiagnosticParseKinds.td
+++ include/clang/Basic/DiagnosticParseKinds.td
@@ -173,6 +173,9 @@
 def warn_gcc_attribute_location : Warning<
   "GCC does not allow an attribute in this position on a function declaration">, 
   InGroup<GccCompat>;
+def warn_gcc_variable_decl_in_for_loop : Warning<
+  "GCC does not allow variable declarations in for loop initializers before "
+  "C99">, InGroup<GccCompat>;
 def warn_attribute_no_decl : Warning<
   "attribute %0 ignored, because it is not attached to a declaration">, 
   InGroup<IgnoredAttributes>;


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