cmd/compile: use correct line number for := (LCOLAS)

- use same local variable name (lno) for line number for LCOLAS everywhere
- remove now unneeded assignment of line number to yylval.i in lexer

Fix per suggestion of mdempsky.

Fixes #13415.

Change-Id: Ie3c7f5681615042a12b81b26724b3a5d8a979c25
Reviewed-on: https://go-review.googlesource.com/17248
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -1253,7 +1253,6 @@ l0:
 		c1 = getc()
 		if c1 == '=' {
 			c = int(LCOLAS)
-			yylval.i = int(lexlineno)
 			goto lx
 		}
 

src/cmd/compile/internal/gc/parser.go

@@ -717,7 +717,7 @@ func (p *parser) simple_stmt(labelOk, rangeOk bool) *Node {
 		return stmt
 
 	case LCOLAS:
-		line := lineno
+		lno := lineno
 		p.next()
 
 		if rangeOk && p.got(LRANGE) {
@@ -746,7 +746,7 @@ func (p *parser) simple_stmt(labelOk, rangeOk bool) *Node {
 			} // it's a colas, so must not re-use an oldname
 			return ts
 		}
-		return colas(lhs, rhs, int32(line))
+		return colas(lhs, rhs, int32(lno))
 
 	default:
 		p.syntax_error("expecting := or = or comma")
@@ -849,6 +849,7 @@ func (p *parser) case_(tswitch *Node) *Node {
 
 		case LCOLAS:
 			// LCASE expr_or_type_list LCOLAS expr ':'
+			lno := lineno
 			p.next()
 			rhs := p.expr()
 
@@ -857,7 +858,7 @@ func (p *parser) case_(tswitch *Node) *Node {
 			// done in casebody()
 			markdcl() // matching popdcl in caseblock
 			stmt := Nod(OXCASE, nil, nil)
-			stmt.List = list1(colas(cases, list1(rhs), int32(p.op)))
+			stmt.List = list1(colas(cases, list1(rhs), int32(lno)))
 
 			p.want(':') // consume ':' after declaring select cases for correct lineno
 			return stmt

test/fixedbugs/issue13415.go

+// errorcheck
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Verify that error message regarding := appears on
+// correct line (and not on the line of the 2nd :=).
+
+package p
+
+func f() {
+    select {
+    case x, x := <-func() chan int { // ERROR "x repeated on left side of :="
+            c := make(chan int)
+            return c
+    }():
+    }
+}