cmd/compile: simplify exporting universal 'error' type

There shouldn't be any problems setting error's "Orig" (underlying)
type to a separate anonymous interface, as this is already how
go/types defines it.

Change-Id: I44e9c4048ffe362ce329e8306632e38b5ccfecff
Reviewed-on: https://go-review.googlesource.com/61790
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Alan Donovan <adonovan@google.com>

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

@@ -601,8 +601,6 @@ func isInlineable(n *Node) bool {
 	return false
 }
 
-var errorInterface *types.Type // lazily initialized
-
 func (p *exporter) typ(t *types.Type) {
 	if t == nil {
 		Fatalf("exporter: nil type")
@@ -654,19 +652,7 @@ func (p *exporter) typ(t *types.Type) {
 		p.qualifiedName(tsym)
 
 		// write underlying type
-		orig := t.Orig
-		if orig == types.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)
+		p.typ(t.Orig)
 
 		// interfaces don't have associated methods
 		if t.Orig.IsInterface() {

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

@@ -385,10 +385,7 @@ func lexinit1() {
 	s := builtinpkg.Lookup("error")
 	types.Errortype = makeErrorInterface()
 	types.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()
+	types.Errortype.Orig = makeErrorInterface()
 	s.Def = asTypesNode(typenod(types.Errortype))
 
 	// We create separate byte and rune types for better error messages