fix: honor Equal methods on non-struct values

deep.go

@@ -203,6 +203,28 @@ func (c *cmp) equals(a, b reflect.Value, level int) {
 		return
 	}
 
+	// Types with an Equal() method, like time.Time or net.IP, only if struct
+	// field is exported (CanInterface)
+	if eqFunc := a.MethodByName("Equal"); eqFunc.IsValid() && eqFunc.CanInterface() {
+		// Handle https://github.com/go-test/deep/issues/15:
+		// Don't call T.Equal if the method is from an embedded struct, like:
+		//   type Foo struct { time.Time }
+		// First, we'll encounter Equal(Ttime, time.Time) but if we pass b
+		// as the 2nd arg we'll panic: "Call using pkg.Foo as type time.Time"
+		// As far as I can tell, there's no way to see that the method is from
+		// time.Time not Foo. So we check the type of the 1st (0) arg and skip
+		// unless it's b type. Later, we'll encounter the time.Time anonymous/
+		// embedded field and then we'll have Equal(time.Time, time.Time).
+		funcType := eqFunc.Type()
+		if funcType.NumIn() == 1 && funcType.In(0) == bType {
+			retVals := eqFunc.Call([]reflect.Value{b})
+			if !retVals[0].Bool() {
+				c.saveDiff(a, b)
+			}
+			return
+		}
+	}
+
 	switch aKind {
 
 	/////////////////////////////////////////////////////////////////////
@@ -221,28 +243,6 @@ func (c *cmp) equals(a, b reflect.Value, level int) {
 			Iterate through the fields (FirstName, LastName), recurse into their values.
 		*/
 
-		// Types with an Equal() method, like time.Time, only if struct field
-		// is exported (CanInterface)
-		if eqFunc := a.MethodByName("Equal"); eqFunc.IsValid() && eqFunc.CanInterface() {
-			// Handle https://github.com/go-test/deep/issues/15:
-			// Don't call T.Equal if the method is from an embedded struct, like:
-			//   type Foo struct { time.Time }
-			// First, we'll encounter Equal(Ttime, time.Time) but if we pass b
-			// as the 2nd arg we'll panic: "Call using pkg.Foo as type time.Time"
-			// As far as I can tell, there's no way to see that the method is from
-			// time.Time not Foo. So we check the type of the 1st (0) arg and skip
-			// unless it's b type. Later, we'll encounter the time.Time anonymous/
-			// embedded field and then we'll have Equal(time.Time, time.Time).
-			funcType := eqFunc.Type()
-			if funcType.NumIn() == 1 && funcType.In(0) == bType {
-				retVals := eqFunc.Call([]reflect.Value{b})
-				if !retVals[0].Bool() {
-					c.saveDiff(a, b)
-				}
-				return
-			}
-		}
-
 		for i := 0; i < a.NumField(); i++ {
 			if aType.Field(i).PkgPath != "" && !CompareUnexportedFields {
 				continue // skip unexported field, e.g. s in type T struct {s string}

equal_method_test.go

@@ -0,0 +1,38 @@
+package deep_test
+
+import (
+	"net"
+	"strings"
+	"testing"
+
+	"github.com/go-test/deep"
+)
+
+type equalFoldString string
+
+func (s equalFoldString) Equal(other equalFoldString) bool {
+	return strings.EqualFold(string(s), string(other))
+}
+
+func TestEqualMethodOnScalar(t *testing.T) {
+	if diff := deep.Equal(equalFoldString("FOO"), equalFoldString("foo")); diff != nil {
+		t.Errorf("case-insensitive strings should be equal: %v", diff)
+	}
+	if diff := deep.Equal(equalFoldString("foo"), equalFoldString("bar")); diff == nil {
+		t.Error("different strings should not be equal")
+	}
+}
+
+func TestEqualMethodOnSlice(t *testing.T) {
+	a := net.ParseIP("1.2.3.4")
+	b := a.To4()
+	if diff := deep.Equal(a, b); diff != nil {
+		t.Errorf("equivalent IPv4 representations should be equal: %v", diff)
+	}
+	if diff := deep.Equal(b, a); diff != nil {
+		t.Errorf("equivalent IPv4 representations should be equal in reverse: %v", diff)
+	}
+	if diff := deep.Equal(a, net.ParseIP("1.2.3.5")); diff == nil {
+		t.Error("different IP addresses should not be equal")
+	}
+}