cmd/compile: fix names of inlined methods from other packages

Previously, an inlined call to wg.Done() in package main would have the
following incorrect symbol name:

    main.(*sync.WaitGroup).Done

This change modifies methodname to return the correct symbol name:

    sync.(*WaitGroup).Done

This fix was suggested by @mdempsky.

Fixes #19467.

Change-Id: I0117838679ac5353789299c618ff8c326712d94d
Reviewed-on: https://go-review.googlesource.com/37866Reviewed-by: Austin Clements <austin@google.com>

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

@@ -1041,16 +1041,12 @@ func methodname(s *Sym, recv *Type) *Sym {
 
 	var p string
 	if star {
-		p = fmt.Sprintf("(*%v).%v", tsym, s)
+		p = fmt.Sprintf("(*%v).%v", tsym.Name, s)
 	} else {
 		p = fmt.Sprintf("%v.%v", tsym, s)
 	}
 
-	if exportname(tsym.Name) {
-		s = lookup(p)
-	} else {
-		s = Pkglookup(p, tsym.Pkg)
-	}
+	s = Pkglookup(p, tsym.Pkg)
 
 	return s
 }

test/fixedbugs/issue19467.dir/mysync.go

+// Copyright 2017 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 mysync
+
+import "runtime"
+
+type WaitGroup struct {
+	Callers []uintptr
+}
+
+func (wg *WaitGroup) Add(x int) {
+	wg.Callers = make([]uintptr, 32)
+	n := runtime.Callers(1, wg.Callers)
+	wg.Callers = wg.Callers[:n]
+}
+
+func (wg *WaitGroup) Done() {
+	wg.Add(-1)
+}

test/fixedbugs/issue19467.dir/z.go

+// Copyright 2017 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 (
+	"./mysync"
+	"log"
+	"runtime"
+)
+
+func main() {
+	var wg mysync.WaitGroup
+	wg.Done()
+	ci := runtime.CallersFrames(wg.Callers)
+	frames := make([]runtime.Frame, 0, 4)
+	for {
+		frame, more := ci.Next()
+		frames = append(frames, frame)
+		if !more {
+			break
+		}
+	}
+	expecting := []string{
+		"mysync.(*WaitGroup).Add",
+		"mysync.(*WaitGroup).Done",
+	}
+	for i := 0; i < 2; i++ {
+		if frames[i].Function != expecting[i] {
+			log.Fatalf("frame %d: got %s, want %s", i, frames[i].Function, expecting[i])
+		}
+	}
+}

test/fixedbugs/issue19467.go

+// rundir -l=4
+
+// Copyright 2017 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 ignored