Make -Wgcc-compat complain about declarations in for loop init statements

[george.burgess.iv]

Summary

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. :)

[george.burgess.iv]

Ping :)

[rsmith]

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

Me either, but given our policy that warning flags just filter out warnings, I'm not sure what other options we have.

[george.burgess.iv]

Thanks!

include/clang/Basic/DiagnosticParseKinds.td

 def err_expected_fn_body : Error<
   "expected function body after function declarator">;
 def warn_attribute_on_function_definition : Warning<
   "GCC does not allow %0 attribute in this position on a function definition">, 
   InGroup<GccCompat>;
 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>;
 def err_ms_attributes_not_enabled : Error<
   "'__declspec' attributes are not enabled; use '-fdeclspec' or "
   "'-fms-extensions' to enable support for __declspec attributes">;
 def err_expected_method_body : Error<"expected method body">;
 def err_declspec_after_virtspec : Error<

lib/Parse/ParseStmt.cpp

Diag(Loc, diag::err_for_range_identifier)
               ? FixItHint::CreateInsertion(Loc, "auto &&")
               : FixItHint());
 
     FirstPart = Actions.ActOnCXXForRangeIdentifier(getCurScope(), Loc, Name,
                                                    attrs, attrs.Range.getEnd());
     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;
     ColonProtectionRAIIObject ColonProtection(*this, MightBeForRangeStmt);
 
     SourceLocation DeclStart = Tok.getLocation(), DeclEnd;
     DeclGroupPtrTy DG = ParseSimpleDeclaration(
         DeclaratorContext::ForContext, DeclEnd, attrs, false,

test/Parser/gcc-for-loop-init-compatibility.c

+// 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@+2{{GCC does not allow variable declarations in for loop initializers before C99}}
+#endif
+  for (int i = 0; i < 10; i++)
+    ;
+}