oneof constraints' required now checks whether scalar fields are empty

internal/evaluator/oneof.go

@@ -33,12 +33,26 @@ func (o oneof) Evaluate(val protoreflect.Value, failFast bool) error {
 }
 
 func (o oneof) EvaluateMessage(msg protoreflect.Message, _ bool) error {
-	if o.Required && msg.WhichOneof(o.Descriptor) == nil {
-		return &errors.ValidationError{Violations: []*validate.Violation{{
-			FieldPath:    string(o.Descriptor.Name()),
-			ConstraintId: "required",
-			Message:      "exactly one field is required in oneof",
-		}}}
+	if !o.Required {
+		return nil
+	}
+	validationErr := &errors.ValidationError{Violations: []*validate.Violation{{
+		FieldPath:    string(o.Descriptor.Name()),
+		ConstraintId: "required",
+		Message:      "exactly one field is required in oneof",
+	}}}
+	presentFieldDescriptor := msg.WhichOneof(o.Descriptor)
+	if presentFieldDescriptor == nil {
+		return validationErr
+	}
+	if presentFieldDescriptor.Kind() == protoreflect.GroupKind || presentFieldDescriptor.Kind() == protoreflect.MessageKind {
+		if !msg.Get(presentFieldDescriptor).Message().IsValid() {
+			return validationErr
+		}
+		return nil
+	}
+	if msg.Get(presentFieldDescriptor).Equal(presentFieldDescriptor.Default()) {
+		return validationErr
 	}
 	return nil
 }

proto/tests/example/v1/validations.proto

@@ -153,3 +153,11 @@ message FieldOfTypeAny {
     expression: "this == this"
   }];
 }
+
+message RequiredOneof {
+  oneof required_oneof {
+    option (buf.validate.oneof).required = true;
+    string fld_1 = 1;
+    google.protobuf.FieldMask fld_2 = 2;
+  }
+}

validator_test.go

@@ -167,3 +167,65 @@ func TestValidator_Validate_FieldOfTypeAny(t *testing.T) {
 	err = val.Validate(msg)
 	require.NoError(t, err)
 }
+
+func TestValidator_RequiredOneof(t *testing.T) {
+	t.Parallel()
+	val, err := New()
+	require.NoError(t, err)
+
+	tests := []struct {
+		msg   *pb.RequiredOneof
+		exErr bool
+	}{
+		{
+			&pb.RequiredOneof{},
+			true,
+		},
+		{
+			&pb.RequiredOneof{
+				RequiredOneof: &pb.RequiredOneof_Fld_2{
+					// nil FieldMask is empty.
+					Fld_2: nil,
+				},
+			},
+			true,
+		},
+		{
+			&pb.RequiredOneof{
+				RequiredOneof: &pb.RequiredOneof_Fld_2{
+					// this passes `required`
+					Fld_2: &fieldmaskpb.FieldMask{},
+				},
+			},
+			false,
+		},
+		{
+			&pb.RequiredOneof{
+				RequiredOneof: &pb.RequiredOneof_Fld_1{
+					// empty string does not pass `required` as a normal field,
+					// and it shouldn't pass it here either.
+					Fld_1: "",
+				},
+			},
+			true,
+		},
+		{
+			&pb.RequiredOneof{
+				RequiredOneof: &pb.RequiredOneof_Fld_1{
+					// passes `required`
+					Fld_1: "foo",
+				},
+			},
+			false,
+		},
+	}
+	for _, test := range tests {
+		test := test
+		err = val.Validate(test.msg)
+		if test.exErr {
+			require.Error(t, err)
+		} else {
+			require.NoError(t, err)
+		}
+	}
+}