cmd/compile: fix line number for implicitly declared method expressions

Method expressions where the method is implicitly declared have no
line number. The Error method of the built-in error type is one such
method.  We leave the line number at the use of the method expression
in this case.

Fixes #29389

Change-Id: I29c64bb47b1a704576abf086599eb5af7b78df53
Reviewed-on: https://go-review.googlesource.com/c/155639
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -439,9 +439,14 @@ func makepartialcall(fn *Node, t0 *types.Type, meth *types.Sym) *Node {
 
 	// Set line number equal to the line number where the method is declared.
 	var m *types.Field
-	if lookdot0(meth, rcvrtype, &m, false) == 1 {
+	if lookdot0(meth, rcvrtype, &m, false) == 1 && m.Pos.IsKnown() {
 		lineno = m.Pos
 	}
+	// Note: !m.Pos.IsKnown() happens for method expressions where
+	// the method is implicitly declared. The Error method of the
+	// built-in error type is one such method.  We leave the line
+	// number at the use of the method expression in this
+	// case. See issue 29389.
 
 	tfn := nod(OTFUNC, nil, nil)
 	tfn.List.Set(structargs(t0.Params(), true))

test/fixedbugs/issue29389.go

@@ -0,0 +1,17 @@
+// compile
+
+// 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.
+
+// Make sure we can correctly compile method expressions
+// where the method is implicitly declared.
+
+package main
+
+import "io"
+
+func main() {
+	err := io.EOF
+	_ = err.Error
+}