go/types: fix TypeString(nil, nil)

The code is meant to return "<nil>", but because of a make([]Type, 8)
call that should be make([]Type, 0, 8), the nil Type happens to
already appear in the array.

Change-Id: I2db140046e52f27db1b0ac84bde2b6680677dd95
Reviewed-on: https://go-review.googlesource.com/16464
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/typestring.go

@@ -71,7 +71,7 @@ func TypeString(typ Type, qf Qualifier) string {
 // The Qualifier controls the printing of
 // package-level objects, and may be nil.
 func WriteType(buf *bytes.Buffer, typ Type, qf Qualifier) {
-	writeType(buf, typ, qf, make([]Type, 8))
+	writeType(buf, typ, qf, make([]Type, 0, 8))
 }
 
 func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
@@ -272,7 +272,7 @@ func writeTuple(buf *bytes.Buffer, tup *Tuple, variadic bool, qf Qualifier, visi
 // The Qualifier controls the printing of
 // package-level objects, and may be nil.
 func WriteSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier) {
-	writeSignature(buf, sig, qf, make([]Type, 8))
+	writeSignature(buf, sig, qf, make([]Type, 0, 8))
 }
 
 func writeSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier, visited []Type) {

src/go/types/typestring_test.go

@@ -148,6 +148,7 @@ func TestQualifiedTypeString(t *testing.T) {
 		this *Package
 		want string
 	}{
+		{nil, nil, "<nil>"},
 		{pT, nil, "p.T"},
 		{pT, p, "T"},
 		{pT, q, "p.T"},