cmd/compile: correctly export underlying type of predecl. error type

Fixes #15920.

Change-Id: I78cd79b91a58d0f7218b80f9445417f4ee071a6e
Reviewed-on: https://go-review.googlesource.com/23606
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -622,6 +622,8 @@ func isInlineable(n *Node) bool {
 	return false
 }
 
+var errorInterface *Type // lazily initialized
+
 func (p *exporter) typ(t *Type) {
 	if t == nil {
 		Fatalf("exporter: nil type")
@@ -673,7 +675,19 @@ func (p *exporter) typ(t *Type) {
 		p.qualifiedName(tsym)
 
 		// write underlying type
-		p.typ(t.Orig)
+		orig := t.Orig
+		if orig == errortype {
+			// The error type is the only predeclared type which has
+			// a composite underlying type. When we encode that type,
+			// make sure to encode the underlying interface rather than
+			// the named type again. See also the comment in universe.go
+			// regarding the errortype and issue #15920.
+			if errorInterface == nil {
+				errorInterface = makeErrorInterface()
+			}
+			orig = errorInterface
+		}
+		p.typ(orig)
 
 		// interfaces don't have associated methods
 		if t.Orig.IsInterface() {

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

@@ -358,9 +358,7 @@ func typeinit() {
 	itable = typPtr(Types[TUINT8])
 }
 
-func lexinit1() {
-	// t = interface { Error() string }
-
+func makeErrorInterface() *Type {
 	rcvr := typ(TSTRUCT)
 	rcvr.StructType().Funarg = FunargRcvr
 	field := newField()
@@ -387,10 +385,18 @@ func lexinit1() {
 	field.Type = f
 	t.SetFields([]*Field{field})
 
+	return t
+}
+
+func lexinit1() {
 	// error type
 	s := Pkglookup("error", builtinpkg)
-	errortype = t
+	errortype = makeErrorInterface()
 	errortype.Sym = s
+	// TODO: If we can prove that it's safe to set errortype.Orig here
+	// than we don't need the special errortype/errorInterface case in
+	// bexport.go. See also issue #15920.
+	// errortype.Orig = makeErrorInterface()
 	s.Def = typenod(errortype)
 
 	// byte alias

src/go/internal/gcimporter/gcimporter_test.go

@@ -400,3 +400,28 @@ func TestIssue15517(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue15920(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+		return
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	if f := compile(t, "testdata", "issue15920.go"); f != "" {
+		defer os.Remove(f)
+	}
+
+	imports := make(map[string]*types.Package)
+	if _, err := Import(imports, "./testdata/issue15920", "."); err != nil {
+		t.Fatal(err)
+	}
+}

src/go/internal/gcimporter/testdata/issue15920.go

@@ -0,0 +1,11 @@
+// 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 p
+
+// The underlying type of Error is the underlying type of error.
+// Make sure we can import this again without problems.
+type Error error
+
+func F() Error { return nil }

test/fixedbugs/issue15920.dir/a.go

@@ -0,0 +1,9 @@
+// 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 Error error
+
+func F() Error { return nil }

test/fixedbugs/issue15920.dir/b.go

@@ -0,0 +1,7 @@
+// 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 b
+
+import _ "./a"

test/fixedbugs/issue15920.go

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