cmd/compile: clean up encoding of method expressions and add test

Fixes #15646.

Change-Id: Ic13d1adc0a358149209195cdb03811eeee506fb8
Reviewed-on: https://go-review.googlesource.com/23052
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1157,9 +1157,9 @@ func (p *exporter) expr(n *Node) {
 		// Special case: name used as local variable in export.
 		// _ becomes ~b%d internally; print as _ for export
 		if n.Sym != nil && n.Sym.Name[0] == '~' && n.Sym.Name[1] == 'b' {
-			// case 0: mapped to ONAME
-			p.op(ONAME)
-			p.bool(true) // indicate blank identifier
+			// case 0: mapped to OPACK
+			p.op(OPACK)
+			p.string("_") // inlined and customized version of p.sym(n)
 			break
 		}
 
@@ -1174,22 +1174,18 @@ func (p *exporter) expr(n *Node) {
 		// but for export, this should be rendered as (*pkg.T).meth.
 		// These nodes have the special property that they are names with a left OTYPE and a right ONAME.
 		if n.Left != nil && n.Left.Op == OTYPE && n.Right != nil && n.Right.Op == ONAME {
-			// case 2: mapped to ONAME
-			p.op(ONAME)
-			// TODO(gri) can we map this case directly to OXDOT
-			//           and then get rid of the bool here?
-			p.bool(false) // indicate non-blank identifier
-			p.typ(n.Left.Type)
+			// case 2: mapped to OXDOT
+			p.op(OXDOT)
+			p.expr(n.Left) // n.Left.Op == OTYPE
 			p.fieldSym(n.Right.Sym, true)
 			break
 		}
 
 		// case 3: mapped to OPACK
-		p.op(OPACK)
-		p.sym(n) // fallthrough inlined here
+		fallthrough
 
 	case OPACK, ONONAME:
-		p.op(op)
+		p.op(OPACK)
 		p.sym(n)
 
 	case OTYPE:
@@ -1508,6 +1504,8 @@ func (p *exporter) fieldSym(s *Sym, short bool) {
 	}
 }
 
+// sym must encode the _ (blank) identifier as a single string "_" since
+// encoding for some nodes is based on this assumption (e.g. ONAME nodes).
 func (p *exporter) sym(n *Node) {
 	s := n.Sym
 	if s.Pkg != nil {

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

@@ -798,15 +798,10 @@ func (p *importer) node() *Node {
 		}
 		return n
 
-	case ONAME:
-		if p.bool() {
-			// "_"
-			// TODO(gri) avoid repeated "_" lookup
-			return mkname(Pkglookup("_", localpkg))
-		}
-		return NodSym(OXDOT, typenod(p.typ()), p.fieldSym())
+	// case ONAME, OPACK, ONONAME:
+	// 	unreachable - mapped to case OPACK below by exporter
 
-	case OPACK, ONONAME:
+	case OPACK:
 		return mkname(p.sym())
 
 	case OTYPE:

test/fixedbugs/issue15646.dir/a.go

+// Copyright 2016 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 a
+
+type T struct{}
+
+func (T) m() string {
+	return "m"
+}
+
+func (*T) mp() string {
+	return "mp"
+}
+
+func F() func(T) string {
+	return T.m // method expression
+}
+
+func Fp() func(*T) string {
+	return (*T).mp // method expression
+}

test/fixedbugs/issue15646.dir/b.go

+// Copyright 2016 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 main
+
+import "./a" // import must succeed
+
+func main() {
+	if a.F()(a.T{}) != "m" {
+		panic(0)
+	}
+	if a.Fp()(nil) != "mp" {
+		panic(1)
+	}
+}

test/fixedbugs/issue15646.go

+// rundir
+
+// Copyright 2016 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.
+
+// Test that method expressions are correctly encoded
+// in binary export data and can be imported again.
+package ignore
\ No newline at end of file