Fix omitempty on aliased types

Instead of doing return, insert an if block to skip emitting zero fields.

This also allows the check to be inserted at any level.

Fixes #376

Emitted code:

```
// MarshalMsg implements msgp.Marshaler
func (z TypeSample) MarshalMsg(b []byte) (o []byte, err error) {
	o = msgp.Require(b, z.Msgsize())
	// check for omitted fields
	zb0001Len := uint32(2)
	var zb0001Mask uint8 /* 2 bits */
	_ = zb0001Mask
	if z.K == 0 {
		zb0001Len--
		zb0001Mask |= 0x1
	}
	if z.V == 0 {
		zb0001Len--
		zb0001Mask |= 0x2
	}
	// variable map header, size zb0001Len
	o = append(o, 0x80|uint8(zb0001Len))
	// Skip if no fields are to be emitted
	if zb0001Len != 0 {
		if (zb0001Mask & 0x1) == 0 { // if not omitted
			// string "k"
			o = append(o, 0xa1, 0x6b)
			o = msgp.AppendUint32(o, z.K)
		}
		if (zb0001Mask & 0x2) == 0 { // if not omitted
			// string "v"
			o = append(o, 0xa1, 0x76)
			o = msgp.AppendUint32(o, z.V)
		}
	}
	return
}
```

If only 1 field, the check is omitted (and there is similar behavior on clearomitted).

_generated/omitempty.go

@@ -54,6 +54,13 @@ type OmitEmpty0 struct {
 	ATime time.Time `msg:"atime,omitempty"`
 }
 
+type TypeSample struct {
+	K uint32 `msg:"k,omitempty"`
+	V uint32 `msg:"v,omitempty"`
+}
+
+type TypeSamples []TypeSample
+
 type (
 	NamedBool    bool
 	NamedInt     int

_generated/omitempty_test.go

@@ -3,6 +3,7 @@ package _generated
 import (
 	"bytes"
 	"io"
+	"reflect"
 	"testing"
 
 	"github.com/tinylib/msgp/msgp"
@@ -285,3 +286,35 @@ func BenchmarkNotOmitEmpty10AllFull(b *testing.B) {
 		}
 	}
 }
+
+func TestTypeAlias(t *testing.T) {
+	value := TypeSamples{TypeSample{}, TypeSample{K: 1, V: 2}}
+	encoded, err := value.MarshalMsg(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var got TypeSamples
+	_, err = got.UnmarshalMsg(encoded)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(value, got) {
+		t.Errorf("UnmarshalMsg got %v want %v", value, got)
+	}
+	var buf bytes.Buffer
+	w := msgp.NewWriter(&buf)
+	err = value.EncodeMsg(w)
+	if err != nil {
+		t.Fatal(err)
+	}
+	w.Flush()
+	got = TypeSamples{}
+	r := msgp.NewReader(&buf)
+	err = got.DecodeMsg(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(value, got) {
+		t.Errorf("UnmarshalMsg got %v want %v", value, got)
+	}
+}

gen/decode.go

@@ -159,7 +159,9 @@ func (d *decodeGen) structAsMap(s *Struct) {
 
 	if oeCount > 0 {
 		d.p.printf("\n// Clear omitted fields.\n")
-		d.p.printf("if %s {\n", bm.notAllSet())
+		if bm.bitlen > 1 {
+			d.p.printf("if %s {\n", bm.notAllSet())
+		}
 		for bitIdx, fieldIdx := range oeEmittedIdx {
 			fieldElem := s.Fields[fieldIdx].FieldElem
 
@@ -172,7 +174,9 @@ func (d *decodeGen) structAsMap(s *Struct) {
 			}
 			d.p.printf("}\n")
 		}
-		d.p.printf("}")
+		if bm.bitlen > 1 {
+			d.p.printf("}")
+		}
 	}
 }
 

gen/encode.go

@@ -3,7 +3,6 @@ package gen
 import (
 	"fmt"
 	"io"
-	"strings"
 
 	"github.com/tinylib/msgp/msgp"
 )
@@ -142,6 +141,7 @@ func (e *encodeGen) structmap(s *Struct) {
 
 	omitempty := s.AnyHasTagPart("omitempty")
 	omitzero := s.AnyHasTagPart("omitzero")
+	var closeZero bool
 	var fieldNVar string
 	if omitempty || omitzero {
 
@@ -175,9 +175,11 @@ func (e *encodeGen) structmap(s *Struct) {
 			return
 		}
 
-		// quick return for the case where the entire thing is empty, but only at the top level
-		if !strings.Contains(s.Varname(), ".") {
-			e.p.printf("\nif %s == 0 { return }", fieldNVar)
+		// Skip block, if no fields are set.
+		if nfields > 1 {
+			e.p.printf("\n// Skip if no fields are to be emitted")
+			e.p.printf("\nif %s != 0 {", fieldNVar)
+			closeZero = true
 		}
 
 	} else {
@@ -226,7 +228,9 @@ func (e *encodeGen) structmap(s *Struct) {
 		if oeField || anField {
 			e.p.print("\n}") // close if statement
 		}
-
+	}
+	if closeZero {
+		e.p.printf("\n}") // close if statement
 	}
 }
 

gen/marshal.go

@@ -3,7 +3,6 @@ package gen
 import (
 	"fmt"
 	"io"
-	"strings"
 
 	"github.com/tinylib/msgp/msgp"
 )
@@ -137,6 +136,7 @@ func (m *marshalGen) mapstruct(s *Struct) {
 
 	omitempty := s.AnyHasTagPart("omitempty")
 	omitzero := s.AnyHasTagPart("omitzero")
+	var closeZero bool
 	var fieldNVar string
 	if omitempty || omitzero {
 
@@ -169,9 +169,11 @@ func (m *marshalGen) mapstruct(s *Struct) {
 			return
 		}
 
-		// quick return for the case where the entire thing is empty, but only at the top level
-		if !strings.Contains(s.Varname(), ".") {
-			m.p.printf("\nif %s == 0 { return }", fieldNVar)
+		// Skip block, if no fields are set.
+		if nfields > 1 {
+			m.p.printf("\n// Skip if no fields are to be emitted")
+			m.p.printf("\nif %s != 0 {", fieldNVar)
+			closeZero = true
 		}
 
 	} else {
@@ -222,7 +224,9 @@ func (m *marshalGen) mapstruct(s *Struct) {
 		if oeField || anField {
 			m.p.printf("\n}") // close if statement
 		}
-
+	}
+	if closeZero {
+		m.p.printf("\n}") // close if statement
 	}
 }
 

gen/unmarshal.go

@@ -151,7 +151,9 @@ func (u *unmarshalGen) mapstruct(s *Struct) {
 	u.p.print("\n}\n}") // close switch and for loop
 	if oeCount > 0 {
 		u.p.printf("\n// Clear omitted fields.\n")
-		u.p.printf("if %s {\n", bm.notAllSet())
+		if bm.bitlen > 1 {
+			u.p.printf("if %s {\n", bm.notAllSet())
+		}
 		for bitIdx, fieldIdx := range oeEmittedIdx {
 			fieldElem := s.Fields[fieldIdx].FieldElem
 
@@ -164,7 +166,9 @@ func (u *unmarshalGen) mapstruct(s *Struct) {
 			}
 			u.p.printf("}\n")
 		}
-		u.p.printf("}")
+		if bm.bitlen > 1 {
+			u.p.printf("}")
+		}
 	}
 }