cmd/compile/internal/syntax: spurious 2nd syntax error in incorrect 'if' header

[griesemer]

Parsing (go test -run Parse -src=x.go)

//+build ignore

package p

func bad() {
	if f()) {
		return
	}
}

results in:

parser_test.go:26: x.go:6:11: syntax error: unexpected ), expecting semicolon or newline
parser_test.go:26: x.go:6:11: syntax error: unexpected f, expecting { after if clause

The problem is due to an incorrectly set up variable when parsing statement headers.

The 2nd error is suppressed when using cmd/compile because it's on the same line.

[griesemer]

See also #11377.

[gopherbot]

Change https://golang.org/cl/87317 mentions this issue: cmd/compile/internal/syntax: don't record semi position if there's none

[griesemer]

Safe for 1.10.