cmd/compile: fix compile failure for lazily resolved shadowed types

If expanding an inline function body required lazily expanding a
package-scoped type whose identifier was shadowed within the function
body, the lazy expansion would instead overwrite the local symbol
definition instead of the package-scoped symbol. This was due to
importsym using s.Def instead of s.PkgDef.

Unfortunately, this is yet another consequence of the current awkward
scope handling code.

Passes toolstash-check.

Fixes #25984.

Change-Id: Ia7033e1749a883e6e979c854d4b12b0b28083dd8
Reviewed-on: https://go-review.googlesource.com/120456
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -89,7 +89,7 @@ func dumpexport(bout *bio.Writer) {
 }
 
 func importsym(ipkg *types.Pkg, pos src.XPos, s *types.Sym, op Op) *Node {
-	n := asNode(s.Def)
+	n := asNode(s.PkgDef())
 	if n == nil {
 		// iimport should have created a stub ONONAME
 		// declaration for all imported symbols. The exception
@@ -100,7 +100,7 @@ func importsym(ipkg *types.Pkg, pos src.XPos, s *types.Sym, op Op) *Node {
 		}
 
 		n = dclname(s)
-		s.Def = asTypesNode(n)
+		s.SetPkgDef(asTypesNode(n))
 		s.Importdef = ipkg
 	}
 	if n.Op != ONONAME && n.Op != op {

src/cmd/compile/internal/types/scope.go

@@ -80,15 +80,24 @@ func IsDclstackValid() bool {
 
 // PkgDef returns the definition associated with s at package scope.
 func (s *Sym) PkgDef() *Node {
+	return *s.pkgDefPtr()
+}
+
+// SetPkgDef sets the definition associated with s at package scope.
+func (s *Sym) SetPkgDef(n *Node) {
+	*s.pkgDefPtr() = n
+}
+
+func (s *Sym) pkgDefPtr() **Node {
 	// Look for outermost saved declaration, which must be the
 	// package scope definition, if present.
 	for _, d := range dclstack {
 		if s == d.sym {
-			return d.def
+			return &d.def
 		}
 	}
 
 	// Otherwise, the declaration hasn't been shadowed within a
 	// function scope.
-	return s.Def
+	return &s.Def
 }

test/fixedbugs/issue25984.dir/p.go

@@ -0,0 +1,15 @@
+// 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.
+
+package p
+
+type m struct {
+	link *m
+}
+
+var head *m
+
+func F(m *int) bool {
+	return head != nil
+}

test/fixedbugs/issue25984.dir/q.go

@@ -0,0 +1,11 @@
+// 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.
+
+package q
+
+import "./p"
+
+func G() {
+	p.F(nil)
+}

test/fixedbugs/issue25984.go

@@ -0,0 +1,7 @@
+// compiledir
+
+// 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.
+
+package ignored