feat: use isReflectNil instead of isEmptyValue to gate Transformers

Mergo handles merges with zero-value fields as follows:

	1. dst: true       src: false      ->     dst: true
	2. dst: nil        src: false      ->     dst: false
	3. dst: nil	   src: true	   ->	  dst: true
	4. dst: false	   src: true 	   ->	  dst: true

In some cases (darccio#166) it may be desirable
to change this default behaviour using a custom Transformer, for example
to not overwrite 'dst' when it's already set to 'false' (or '0' etc).
Unfortunately, in such cases (when dst holds a zero-value) no Transformers can
be applied due to the way these are gated (darccio#166).

Here we are changing how transformers are gated by using the isReflectNil function
insted of isEmptyValue.

Note that:
- this change doesn't affect the default behaviour of mergo, as it only affects
  how transformers are gated
- since custom transformers need to match data types in order to execute,
  this shouldn't affect any existing transformers either

.travis.yml

@@ -1,4 +1,12 @@
 language: go
+
+go:
+- "1.13"
+- "1.14"
+- "1.15"
+- "1.16"
+- "1.17"
+
 arch:
     - amd64
     - ppc64le

merge.go

@@ -79,7 +79,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		visited[h] = &visit{addr, typ, seen}
 	}
 
-	if config.Transformers != nil && !isEmptyValue(dst) {
+	if config.Transformers != nil && !isReflectNil(dst) {
 		if fn := config.Transformers.Transformer(dst.Type()); fn != nil {
 			err = fn(dst, src)
 			return

mergo_test.go

@@ -878,6 +878,10 @@ type structWithBoolPointer struct {
 	C *bool
 }
 
+type structWithIntPointer struct {
+	C *int
+}
+
 func TestBooleanPointer(t *testing.T) {
 	bt, bf := true, false
 	src := structWithBoolPointer{
@@ -938,3 +942,130 @@ func TestMergeSlicesIsNotSupported(t *testing.T) {
 		t.Errorf("expected %q, got %q", mergo.ErrNotSupported, err)
 	}
 }
+
+func Bool(v bool) *bool { return &v }
+func Int(i int) *int    { return &i }
+
+type customTransformer struct{}
+
+func (t customTransformer) Transformer(typ reflect.Type) func(dst, src reflect.Value) error {
+	if typ == reflect.TypeOf(Bool(false)) || typ == reflect.TypeOf(Int(0)) {
+		return func(dst, src reflect.Value) error {
+			if dst.CanSet() {
+				if src.IsNil() {
+					src.Set(dst)
+				}
+			}
+			return nil
+		}
+	}
+	return nil
+}
+
+func TestBooleanPointerWithTransformer(t *testing.T) {
+	testCases := []struct {
+		name     string
+		src      *bool
+		dst      *bool
+		expected *bool
+	}{
+		{
+			name:     "do not overwrite if dst is not empty (true)",
+			src:      Bool(false),
+			dst:      Bool(true),
+			expected: Bool(true),
+		},
+		{
+			name:     "do not overwrite if dst is not empty (false)",
+			src:      Bool(true),
+			dst:      Bool(false),
+			expected: Bool(false),
+		},
+		{
+			name:     "overwrite if dst is empty (true)",
+			src:      Bool(true),
+			expected: Bool(true),
+		},
+		{
+			name:     "overwrite if dst is empty (false)",
+			src:      Bool(false),
+			expected: Bool(false),
+		},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			var src, dst structWithBoolPointer
+			if tc.src != nil {
+				src = structWithBoolPointer{
+					C: tc.src,
+				}
+			}
+			if tc.dst != nil {
+				dst = structWithBoolPointer{
+					C: tc.dst,
+				}
+			}
+
+			if err := mergo.Merge(&dst, src, mergo.WithTransformers(customTransformer{})); err != nil {
+				t.Error(err)
+			}
+			if *dst.C != *tc.expected {
+				t.Errorf("expected %v, got %v", *tc.expected, *dst.C)
+			}
+		})
+	}
+}
+
+func TestIntPointerWithTransformer(t *testing.T) {
+	testCases := []struct {
+		name     string
+		src      *int
+		dst      *int
+		expected *int
+	}{
+		{
+			name:     "do not overwrite if dst is not empty (10)",
+			src:      Int(100),
+			dst:      Int(10),
+			expected: Int(10),
+		},
+		{
+			name:     "do not overwrite if dst is not empty (0)",
+			src:      Int(100),
+			dst:      Int(0),
+			expected: Int(0),
+		},
+		{
+			name:     "overwrite if dst is empty (100)",
+			src:      Int(100),
+			expected: Int(100),
+		},
+		{
+			name:     "overwrite if dst is empty (100)",
+			src:      Int(100),
+			expected: Int(100),
+		},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			var src, dst structWithIntPointer
+			if tc.src != nil {
+				src = structWithIntPointer{
+					C: tc.src,
+				}
+			}
+			if tc.dst != nil {
+				dst = structWithIntPointer{
+					C: tc.dst,
+				}
+			}
+
+			if err := mergo.Merge(&dst, src, mergo.WithTransformers(customTransformer{})); err != nil {
+				t.Error(err)
+			}
+			if *dst.C != *tc.expected {
+				t.Errorf("expected %v, got %v", *tc.expected, *dst.C)
+			}
+		})
+	}
+}