cmd/compile: use key position for key:val elements in composite literals

Fixes #24339.

Change-Id: Ie47764fed27f76b480834b1fdbed0512c94831d9
Reviewed-on: https://go-review.googlesource.com/100457
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -569,7 +569,8 @@ func (p *noder) expr(expr syntax.Expr) *Node {
 		lineno = p.makeXPos(expr.Rbrace)
 		return n
 	case *syntax.KeyValueExpr:
-		return p.nod(expr, OKEY, p.expr(expr.Key), p.wrapname(expr.Value, p.expr(expr.Value)))
+		// use position of expr.Key rather than of expr (which has position of ':')
+		return p.nod(expr.Key, OKEY, p.expr(expr.Key), p.wrapname(expr.Value, p.expr(expr.Value)))
 	case *syntax.FuncLit:
 		return p.funcLit(expr)
 	case *syntax.ParenExpr:

test/fixedbugs/issue24339.go

+// errorcheck
+
+// Copyright 2018 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.
+
+package p
+
+// Use a diffent line number for each token so we can
+// check that the error message appears at the correct
+// position.
+var _ = struct{}{ /*line :20:1*/foo /*line :21:1*/: /*line :22:1*/0 }
+
+
+
+
+
+
+
+// ERROR "unknown field 'foo'"
\ No newline at end of file