From b75d2f250836b95028d2bfac2220cef5ccf08837 Mon Sep 17 00:00:00 2001
From: Jim Slattery <jim.slattery@rightscale.com>
Date: Thu, 25 Feb 2021 19:38:56 -0500
Subject: [PATCH] Allow WithTransform function to accept a nil value (#422)

---
 matchers/with_transform.go      | 20 +++++++++++++++-----
 matchers/with_transform_test.go |  8 +++++++-
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/matchers/with_transform.go b/matchers/with_transform.go
index 8e58d8a0f..a4db310b6 100644
--- a/matchers/with_transform.go
+++ b/matchers/with_transform.go
@@ -40,15 +40,25 @@ func NewWithTransformMatcher(transform interface{}, matcher types.GomegaMatcher)
 }
 
 func (m *WithTransformMatcher) Match(actual interface{}) (bool, error) {
-	// return error if actual's type is incompatible with Transform function's argument type
-	actualType := reflect.TypeOf(actual)
-	if !actualType.AssignableTo(m.transformArgType) {
-		return false, fmt.Errorf("Transform function expects '%s' but we have '%s'", m.transformArgType, actualType)
+	// prepare a parameter to pass to the Transform function
+	var param reflect.Value
+	{
+		if actual != nil {
+			// return error if actual's type is incompatible with Transform function's argument type
+			actualType := reflect.TypeOf(actual)
+			if !actualType.AssignableTo(m.transformArgType) {
+				return false, fmt.Errorf("Transform function expects '%s' but we have '%s'", m.transformArgType, actualType)
+			}
+			param = reflect.ValueOf(actual)
+		} else {
+			// make a nil value of the expected type
+			param = reflect.New(m.transformArgType).Elem()
+		}
 	}
 
 	// call the Transform function with `actual`
 	fn := reflect.ValueOf(m.Transform)
-	result := fn.Call([]reflect.Value{reflect.ValueOf(actual)})
+	result := fn.Call([]reflect.Value{param})
 	m.transformedValue = result[0].Interface() // expect exactly one value
 
 	return m.Matcher.Match(m.transformedValue)
diff --git a/matchers/with_transform_test.go b/matchers/with_transform_test.go
index 5fcc949ae..858039e0d 100644
--- a/matchers/with_transform_test.go
+++ b/matchers/with_transform_test.go
@@ -51,7 +51,13 @@ var _ = Describe("WithTransformMatcher", func() {
 		Expect(S{1, "hi"}).To(WithTransform(transformer, Equal("hi")))
 
 		// transform expects interface
-		errString := func(e error) string { return e.Error() }
+		errString := func(e error) string {
+			if e == nil {
+				return "<nil>"
+			}
+			return e.Error()
+		}
+		Expect(nil).To(WithTransform(errString, Equal("<nil>")), "handles nil actual values")
 		Expect(errors.New("abc")).To(WithTransform(errString, Equal("abc")))
 	})