cmd/compile: generate interface method expression wrapper for error.Error

A prior optimization (https://golang.org/cl/106175) removed the
generation of unnecessary method expression wrappers, but also
eliminated the generation of the wrapper for error.Error which
was still required.

Special-case error type in the optimization.

Fixes #29304.

Change-Id: I54c8afc88a2c6d1906afa2d09c68a0a3f3e2f1e3
Reviewed-on: https://go-review.googlesource.com/c/154578Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -1517,8 +1517,9 @@ func genwrapper(rcvr *types.Type, method *types.Field, newnam *types.Sym) {
 		return
 	}
 
-	// Only generate I.M wrappers for I in I's own package.
-	if rcvr.IsInterface() && rcvr.Sym != nil && rcvr.Sym.Pkg != localpkg {
+	// Only generate I.M wrappers for I in I's own package
+	// but keep doing it for error.Error (was issue #29304).
+	if rcvr.IsInterface() && rcvr.Sym != nil && rcvr.Sym.Pkg != localpkg && rcvr != types.Errortype {
 		return
 	}
 

test/fixedbugs/issue29304.go

+// run
+
+// 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.
+
+// Verify that relocation target go.builtin.error.Error
+// is defined and the code links and runs correctly.
+
+package main
+
+import "errors"
+
+func main() {
+	err := errors.New("foo")
+	if error.Error(err) != "foo" {
+		panic("FAILED")
+	}
+}