cmd/compile: use fake package for allocating autos

Make sure auto names don't conflict with function names. Before this CL,
we confused name a.len (the len field of the slice a) with a.len (the function
len declared on a).

Fixes #15961

Change-Id: I14913de697b521fb35db9a1b10ba201f25d552bb
Reviewed-on: https://go-review.googlesource.com/23789
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -156,6 +156,8 @@ var Debug_typeassert int
 
 var localpkg *Pkg // package being compiled
 
+var autopkg *Pkg // fake package for allocating auto variables
+
 var importpkg *Pkg // package being imported
 
 var itabpkg *Pkg // fake pkg for itab entries

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

@@ -108,6 +108,8 @@ func Main() {
 
 	localpkg = mkpkg("")
 	localpkg.Prefix = "\"\""
+	autopkg = mkpkg("")
+	autopkg.Prefix = "\"\""
 
 	// pseudo-package, for scoping
 	builtinpkg = mkpkg("go.builtin")

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

@@ -4378,7 +4378,7 @@ func (e *ssaExport) SplitStruct(name ssa.LocalSlot, i int) ssa.LocalSlot {
 // namedAuto returns a new AUTO variable with the given name and type.
 func (e *ssaExport) namedAuto(name string, typ ssa.Type) ssa.GCNode {
 	t := typ.(*Type)
-	s := Lookup(name)
+	s := &Sym{Name: name, Pkg: autopkg}
 	n := Nod(ONAME, nil, nil)
 	s.Def = n
 	s.Def.Used = true

test/fixedbugs/issue15961.go

@@ -0,0 +1,21 @@
+// compile
+
+// 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 y
+
+type symSet []int
+
+//go:noinline
+func (s symSet) len() (r int) {
+	return 0
+}
+
+func f(m map[int]symSet) {
+	var symSet []int
+	for _, x := range symSet {
+		m[x] = nil
+	}
+}