Improve message from AssignableToTypeOf when expected value is nil (#281

)

* AssignableToTypeOf returns false without error

if exactly one of actual or expected is nil.

Fixes #280.

Signed-off-by: Rowan Jacobs <rojacobs@pivotal.io>

* error when actual is not nil and expected is nil

Signed-off-by: Rowan Jacobs <rojacobs@pivotal.io>

matchers/assignable_to_type_of_matcher.go

@@ -12,8 +12,12 @@ type AssignableToTypeOfMatcher struct {
 }
 
 func (matcher *AssignableToTypeOfMatcher) Match(actual interface{}) (success bool, err error) {
-	if actual == nil || matcher.Expected == nil {
+	if actual == nil && matcher.Expected == nil {
 		return false, fmt.Errorf("Refusing to compare <nil> to <nil>.\nBe explicit and use BeNil() instead.  This is to avoid mistakes where both sides of an assertion are erroneously uninitialized.")
+	} else if matcher.Expected == nil {
+		return false, fmt.Errorf("Refusing to compare type to <nil>.\nBe explicit and use BeNil() instead.  This is to avoid mistakes where both sides of an assertion are erroneously uninitialized.")
+	} else if actual == nil {
+		return false, nil
 	}
 
 	actualType := reflect.TypeOf(actual)

matchers/assignable_to_type_of_matcher_test.go

@@ -26,5 +26,21 @@ var _ = Describe("AssignableToTypeOf", func() {
 			Expect(success).Should(BeFalse())
 			Expect(err).Should(HaveOccurred())
 		})
+
+		Context("When actual is nil and expected is not nil", func() {
+			It("should return false without error", func() {
+				success, err := (&AssignableToTypeOfMatcher{Expected: 17}).Match(nil)
+				Expect(success).Should(BeFalse())
+				Expect(err).ShouldNot(HaveOccurred())
+			})
+		})
+
+		Context("When actual is not nil and expected is nil", func() {
+			It("should error", func() {
+				success, err := (&AssignableToTypeOfMatcher{Expected: nil}).Match(17)
+				Expect(success).Should(BeFalse())
+				Expect(err).Should(HaveOccurred())
+			})
+		})
 	})
 })