cmd/compile: more accurate position for select case error message

Fixes #25958.

Change-Id: I1f4808a70c20334ecfc4eb1789f5389d94dcf00e
Reviewed-on: https://go-review.googlesource.com/119755
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -33,7 +33,15 @@ func typecheckselect(sel *Node) {
 			ncase.List.Set(nil)
 			switch n.Op {
 			default:
-				yyerrorl(n.Pos, "select case must be receive, send or assign recv")
+				pos := n.Pos
+				if n.Op == ONAME {
+					// We don't have the right position for ONAME nodes (see #15459 and
+					// others). Using ncase.Pos for now as it will provide the correct
+					// line number (assuming the expression follows the "case" keyword
+					// on the same line). This matches the approach before 1.10.
+					pos = ncase.Pos
+				}
+				yyerrorl(pos, "select case must be receive, send or assign recv")
 
 			// convert x = <-c into OSELRECV(x, <-c).
 			// remove implicit conversions; the eventual assignment

test/fixedbugs/issue25958.go

@@ -0,0 +1,17 @@
+// 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
+
+// Verify that the "must be receive" error for "case done:" appears
+// on the line of the case clause, not the line of the done declaration.
+
+func f(done chan struct{}) {
+	select {
+	case done: // ERROR "must be receive", "not used"
+	case (chan struct{})(done): // ERROR "must be receive"
+	}
+}