Use reflection for nil error comparison

Author: Geish

.travis.yml

@@ -8,7 +8,6 @@ go:
   - 1.9
   - "1.10"
   - 1.11
-  - tip
 
 env: GO15VENDOREXPERIMENT=1
 script: go test -v ./...

assert/error.go

@@ -1,5 +1,7 @@
 package assert
 
+import "reflect"
+
 // AssertableError represents an under-test error that's expected to meet
 // certain criteria.
 type AssertableError interface {
@@ -34,7 +36,8 @@ func (err ErrorString) Error() string {
 }
 
 func (actual *assertableError) Equals(expected error) ValueAssertionResult {
-	if expected == nil {
+	// Allow reflect to check for nil expected error as that object could have been loaded from JSON file (for DDT)
+	if expected == nil || (reflect.ValueOf(expected).Kind() == reflect.Ptr && reflect.ValueOf(expected).IsNil()) {
 		return actual.IsNil()
 	}
 	if actual.value == nil {
@@ -50,14 +53,14 @@ func (actual *assertableError) Equals(expected error) ValueAssertionResult {
 }
 
 func (actual *assertableError) IsNil() ValueAssertionResult {
-	if actual.value != nil {
+	if actual.value != nil { // no reflection here as we want to verify that the interface itself is not nil
 		actual.testContext.decoratedErrorf("Actual error was not <nil>.\nActual: %v\n", actual.value)
 	}
 	return &valueAssertionResult{bool: actual.value == nil, actual: actual.value, expected: nil}
 }
 
 func (actual *assertableError) IsNotNil() ValueAssertionResult {
-	if actual.value == nil {
+	if actual.value == nil { // no reflection here as we want to verify that the interface itself is nil
 		actual.testContext.decoratedErrorf("Actual error was <nil>.\n")
 	}
 	return &valueAssertionResult{bool: actual.value != nil, actual: actual.value, expected: &anyOtherValue{}}

assert/error_test.go

@@ -6,12 +6,14 @@ import (
 )
 
 func ExampleAssertableError_Equals_pass() {
+	var nilError *ErrorString
 	cases := []struct {
 		id       string
 		actual   error
 		expected error
 	}{
-		{"both are nil", nil, nil},
+		{"nil interfaces", nil, nil},
+		{"nil interface and nil pointer", nil, nilError},
 		{"same type and message", errors.New("foo"), errors.New("foo")},
 		{"different struct types, same message", ErrorString("foo"), errors.New("foo")},
 	}
@@ -22,7 +24,8 @@ func ExampleAssertableError_Equals_pass() {
 		}
 	}
 	// Output:
-	// Passed: both are nil
+	// Passed: nil interfaces
+	// Passed: nil interface and nil pointer
 	// Passed: same type and message
 	// Passed: different struct types, same message
 }