Fix bug with nil fmt.Stringers (#30)

Author: mattdee123

cmp/compare_test.go

@@ -79,6 +79,7 @@ func TestDiff(t *testing.T) {
 tests = append(tests, transformerTests()...)
 tests = append(tests, embeddedTests()...)
 tests = append(tests, methodTests()...)
+tests = append(tests, formatTests()...)
 tests = append(tests, project1Tests()...)
 tests = append(tests, project2Tests()...)
 tests = append(tests, project3Tests()...)
@@ -1361,6 +1362,21 @@ func methodTests() []test {
 }}
 }
 
+func formatTests() []test {
+const label = "Format/"
+return []test{
+{
+label: label + "NilStringer",
+x: new(fmt.Stringer),
+y: nil,
+wantDiff: `
+:
+-: &<nil>
++: <non-existent>`,
+},
+}
+}
+
 func project1Tests() []test {
 const label = "Project1"
 

cmp/internal/value/format.go

@@ -44,7 +44,7 @@ func formatAny(v reflect.Value, conf formatConfig, visited map[uintptr]bool) str
 return "<non-existent>"
 }
 if conf.useStringer && v.Type().Implements(stringerIface) {
-if v.Kind() == reflect.Ptr && v.IsNil() {
+if (v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface) && v.IsNil() {
 return "<nil>"
 }
 return fmt.Sprintf("%q", v.Interface().(fmt.Stringer).String())