internal/impl: weak field bugfixes

Fix a reversed error check in impl.Export{}.WeakNil.

Check to see if we have a type for the weak field on marshal/size.

Treat a typed nil valued in XXX_Weak as not indicating presence for
the field.

Change-Id: Id667ac7eb4f53236be9e181017082bd8cd21d115
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/198717
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/proto/weak_test.go b/proto/weak_test.go
new file mode 100644
index 0000000..992db0d
--- /dev/null
+++ b/proto/weak_test.go
@@ -0,0 +1,83 @@
+// Copyright 2019 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 proto_test
+
+import (
+	"testing"
+
+	"google.golang.org/protobuf/internal/encoding/pack"
+	"google.golang.org/protobuf/internal/flags"
+	"google.golang.org/protobuf/proto"
+
+	testpb "google.golang.org/protobuf/internal/testprotos/test"
+	weakpb "google.golang.org/protobuf/internal/testprotos/test/weak1"
+)
+
+func TestWeak(t *testing.T) {
+	if !flags.ProtoLegacy {
+		t.SkipNow()
+	}
+
+	m := new(testpb.TestWeak)
+	b := pack.Message{
+		pack.Tag{1, pack.BytesType}, pack.LengthPrefix(pack.Message{
+			pack.Tag{1, pack.VarintType}, pack.Varint(1000),
+		}),
+		pack.Tag{2, pack.BytesType}, pack.LengthPrefix(pack.Message{
+			pack.Tag{1, pack.VarintType}, pack.Varint(2000),
+		}),
+	}.Marshal()
+	if err := proto.Unmarshal(b, m); err != nil {
+		t.Errorf("Unmarshal error: %v", err)
+	}
+
+	mw := m.GetWeakMessage1().(*weakpb.WeakImportMessage1)
+	if mw.GetA() != 1000 {
+		t.Errorf("m.WeakMessage1.a = %d, want %d", mw.GetA(), 1000)
+	}
+
+	if len(m.ProtoReflect().GetUnknown()) == 0 {
+		t.Errorf("m has no unknown fields, expected at least something")
+	}
+
+	if n := proto.Size(m); n != len(b) {
+		t.Errorf("Size() = %d, want %d", n, len(b))
+	}
+
+	b2, err := proto.Marshal(m)
+	if err != nil {
+		t.Errorf("Marshal error: %v", err)
+	}
+	if len(b2) != len(b) {
+		t.Errorf("len(Marshal) = %d, want %d", len(b2), len(b))
+	}
+}
+
+func TestWeakNil(t *testing.T) {
+	if !flags.ProtoLegacy {
+		t.SkipNow()
+	}
+
+	m := new(testpb.TestWeak)
+	if v, ok := m.GetWeakMessage1().(*weakpb.WeakImportMessage1); !ok || v != nil {
+		t.Errorf("m.GetWeakMessage1() = type %[1]T(%[1]v), want (*weakpb.WeakImportMessage1)", v)
+	}
+}
+
+func TestWeakMarshalNil(t *testing.T) {
+	if !flags.ProtoLegacy {
+		t.SkipNow()
+	}
+
+	m := new(testpb.TestWeak)
+	m.SetWeakMessage1(nil)
+	if b, err := proto.Marshal(m); err != nil || len(b) != 0 {
+		t.Errorf("Marshal(weak field set to nil) = [%x], %v; want [], nil", b, err)
+	}
+	m.SetWeakMessage1((*weakpb.WeakImportMessage1)(nil))
+	if b, err := proto.Marshal(m); err != nil || len(b) != 0 {
+		t.Errorf("Marshal(weak field set to typed nil) = [%x], %v; want [], nil", b, err)
+	}
+}