add typed error assertions

app_test.go

@@ -1002,6 +1002,9 @@ func TestRequiredFlagAppRunBehavior(t *testing.T) {
 			if test.expectedAnError && err == nil {
 				t.Errorf("expected an error, but there was none")
 			}
+			if _, ok := err.(requiredFlagsErr); test.expectedAnError && !ok {
+				t.Errorf("expected a requiredFlagsErr, but got: %s", err)
+			}
 			if !test.expectedAnError && err != nil {
 				t.Errorf("did not expected an error, but there was one: %s", err)
 			}

context.go

@@ -287,7 +287,29 @@ func normalizeFlags(flags []Flag, set *flag.FlagSet) error {
 	return nil
 }
 
-func checkRequiredFlags(flags []Flag, set *flag.FlagSet) error {
+type requiredFlagsErr interface {
+	error
+	getMissingFlags() []string
+}
+
+type errRequiredFlags struct {
+	missingFlags []string
+}
+
+func (e *errRequiredFlags) Error() string {
+	numberOfMissingFlags := len(e.missingFlags)
+	if numberOfMissingFlags == 1 {
+		return fmt.Sprintf("Required flag %q not set", e.missingFlags[0])
+	}
+	joinedMissingFlags := strings.Join(e.missingFlags, ", ")
+	return fmt.Sprintf("Required flags %q not set", joinedMissingFlags)
+}
+
+func (e *errRequiredFlags) getMissingFlags() []string {
+	return e.missingFlags
+}
+
+func checkRequiredFlags(flags []Flag, set *flag.FlagSet) requiredFlagsErr {
 	visited := make(map[string]bool)
 	set.Visit(func(f *flag.Flag) {
 		visited[f.Name] = true
@@ -303,13 +325,8 @@ func checkRequiredFlags(flags []Flag, set *flag.FlagSet) error {
 		}
 	}
 
-	numberOfMissingFlags := len(missingFlags)
-	if numberOfMissingFlags == 1 {
-		return fmt.Errorf("Required flag %q not set", missingFlags[0])
-	}
-	if numberOfMissingFlags >= 2 {
-		joinedMissingFlags := strings.Join(missingFlags, ", ")
-		return fmt.Errorf("Required flags %q not set", joinedMissingFlags)
+	if len(missingFlags) != 0 {
+		return &errRequiredFlags{missingFlags: missingFlags}
 	}
 
 	return nil