tools/gn: provide better error message for bogus binary expressions

tools/gn/parser.cc

Index: tools/gn/parser.cc
diff --git a/tools/gn/parser.cc b/tools/gn/parser.cc
index 651c0f88d5e8f0e965117764fe6f4547acbc5852..df027ca91808e9fc2b0d800eb3a8904aa80da407 100644
--- a/tools/gn/parser.cc
+++ b/tools/gn/parser.cc
@@ -388,9 +388,9 @@ scoped_ptr<ParseNode> Parser::BinaryOperator(scoped_ptr<ParseNode> left,
   scoped_ptr<ParseNode> right =
       ParseExpression(expressions_[token.type()].precedence + 1);
   if (!right) {
-    *err_ =
-        Err(token,
-            "Expected right hand side for '" + token.value().as_string() + "'");
+    if (!has_error())

[scottmg, 2015/04/07 19:49:52]

nit; braces for multiline if.

[mdempsky, 2015/04/07 19:58:44]

Done.

+      *err_ = Err(token, "Expected right hand side for '" +
+                             token.value().as_string() + "'");

[scottmg, 2015/04/07 19:49:52]

is there a test that hits this code path now? if yes, or with that lgtm.

[mdempsky, 2015/04/07 19:58:44]

Yes, DoExpressionErrorTest("(a +", 1, 4) in Parser.ParenExpression hits this
case.  (ParseExpression(int) returns nullptr without setting an error when
called at EOF.)

     return scoped_ptr<ParseNode>();
   }
   scoped_ptr<BinaryOpNode> binary_op(new BinaryOpNode);