Revert "Support Go 1.13 error chains in Cause (#215)"

This reverts commit 49f8f61.

errors.go

@@ -260,3 +260,29 @@ func (w *withMessage) Format(s fmt.State, verb rune) {
 		io.WriteString(s, w.Error())
 	}
 }
+
+// Cause returns the underlying cause of the error, if possible.
+// An error value has a cause if it implements the following
+// interface:
+//
+//     type causer interface {
+//            Cause() error
+//     }
+//
+// If the error does not implement Cause, the original error will
+// be returned. If the error is nil, nil will be returned without further
+// investigation.
+func Cause(err error) error {
+	type causer interface {
+		Cause() error
+	}
+
+	for err != nil {
+		cause, ok := err.(causer)
+		if !ok {
+			break
+		}
+		err = cause.Cause()
+	}
+	return err
+}

go113.go

@@ -36,36 +36,3 @@ func As(err error, target interface{}) bool { return stderrors.As(err, target) }
 func Unwrap(err error) error {
 	return stderrors.Unwrap(err)
 }
-
-// Cause recursively unwraps an error chain and returns the underlying cause of
-// the error, if possible. There are two ways that an error value may provide a
-// cause. First, the error may implement the following interface:
-//
-//     type causer interface {
-//            Cause() error
-//     }
-//
-// Second, the error may return a non-nil value when passed as an argument to
-// the Unwrap function. This makes Cause forwards-compatible with Go 1.13 error
-// chains.
-//
-// If an error value satisfies both methods of unwrapping, Cause will use the
-// causer interface.
-//
-// If the error is nil, nil will be returned without further investigation.
-func Cause(err error) error {
-	type causer interface {
-		Cause() error
-	}
-
-	for err != nil {
-		if cause, ok := err.(causer); ok {
-			err = cause.Cause()
-		} else if unwrapped := Unwrap(err); unwrapped != nil {
-			err = unwrapped
-		} else {
-			break
-		}
-	}
-	return err
-}

go113_test.go

@@ -9,29 +9,7 @@ import (
 	"testing"
 )
 
-func TestCauseErrorChainCompat(t *testing.T) {
-	err := stderrors.New("the cause!")
-
-	// Wrap error using the standard library
-	wrapped := fmt.Errorf("wrapped with stdlib: %w", err)
-	if Cause(wrapped) != err {
-		t.Errorf("Cause does not support Go 1.13 error chains")
-	}
-
-	// Wrap in another layer using pkg/errors
-	wrapped = WithMessage(wrapped, "wrapped with pkg/errors")
-	if Cause(wrapped) != err {
-		t.Errorf("Cause does not support Go 1.13 error chains")
-	}
-
-	// Wrap in another layer using the standard library
-	wrapped = fmt.Errorf("wrapped with stdlib: %w", wrapped)
-	if Cause(wrapped) != err {
-		t.Errorf("Cause does not support Go 1.13 error chains")
-	}
-}
-
-func TestWrapErrorChainCompat(t *testing.T) {
+func TestErrorChainCompat(t *testing.T) {
 	err := stderrors.New("error that gets wrapped")
 	wrapped := Wrap(err, "wrapped up")
 	if !stderrors.Is(wrapped, err) {