gc: don't emit pkgpath for error type.

Fixes #2660.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5557060

src/cmd/gc/reflect.c

@@ -357,7 +357,7 @@ dextratype(Sym *sym, int off, Type *t, int ptroff)
 	s = sym;
 	if(t->sym) {
 		ot = dgostringptr(s, ot, t->sym->name);
-		if(t != types[t->etype])
+		if(t != types[t->etype] && t != errortype)
 			ot = dgopkgpath(s, ot, t->sym->pkg);
 		else
 			ot = dgostringptr(s, ot, nil);

src/pkg/reflect/all_test.go

@@ -1364,8 +1364,19 @@ func TestFieldByName(t *testing.T) {
 }
 
 func TestImportPath(t *testing.T) {
-	if path := TypeOf(&base64.Encoding{}).Elem().PkgPath(); path != "encoding/base64" {
+	tests := []struct {
-		t.Errorf(`TypeOf(&base64.Encoding{}).Elem().PkgPath() = %q, want "encoding/base64"`, path)
+		t    Type
+		path string
+	}{
+		{TypeOf(&base64.Encoding{}).Elem(), "encoding/base64"},
+		{TypeOf(uint(0)), ""},
+		{TypeOf(map[string]int{}), ""},
+		{TypeOf((*error)(nil)).Elem(), ""},
+	}
+	for _, test := range tests {
+		if path := test.t.PkgPath(); path != test.path {
+			t.Errorf("%v.PkgPath() = %q, want %q", test.t, path, test.path)
+		}
 	}
 }
 

src/pkg/reflect/type.go

@@ -69,7 +69,7 @@ type Type interface {
 
 	// PkgPath returns the type's package path.
 	// The package path is a full package import path like "encoding/base64".
-	// PkgPath returns an empty string for unnamed types.
+	// PkgPath returns an empty string for unnamed or predeclared types.
 	PkgPath() string
 
 	// Size returns the number of bytes needed to store