I'm testing go1.5rc1 on linux/amd64, and I've come across an issue that prevents the compilation of an empty yacc grammar.

Steps to reproduce

Create a file named main.y with the following contents:

%{
package main

type yyLex struct{}
func (x *yyLex) Lex(yylval *yySymType) int { return 0 }
func (x *yyLex) Error(str string) {}

func main() {}
%}

%union {
}

%%

top: /* empty grammar */

%%

Run the following commands:

$ go tool yacc -o main.go main.y
$ go build main.go

Expected results

The code should compile without errors.

Actual results

Compilation fails with the error message:

yaccpar:161[main.go:253]: yyDollar declared and not used

Additional information

The compilation error occurs in the Parse(yylex yyLexer) int method on the *yyParserImpl type. Line 253 of the generated main.go file defines a variable var yyDollar []yySymType, which is never referenced by the rest of the auto-generated method.

I've followed the same steps on Go 1.4, and checked that they produce the expected result. In fact, I believe the issue originated with commit 2e78447, which first introduced the yyDollar variable.