tweak jwt error

Changes-v3.md

@@ -32,6 +32,9 @@ These are changes that are incompatible with the v2.x.x version.
 * Validation used to work for `iat`, `nbf`, `exp` fields where these fields were
   set to the explicit time.Time{} zero value, but now the _presence_ of these fields matter.
 
+* `jwt.ErrInvalidJWT` has been renamed to `jwt.UnknownPayloadTypeError` to better reflect
+  what the error means
+
 ## JWS
 
 * Iterators have been completely removed.

jwt/errors.go

@@ -1,9 +1,19 @@
 package jwt
 
 import (
+	"errors"
 	"fmt"
 )
 
+var errUnknownPayloadType = errors.New(`unknown payload type (payload is not JWT?)`)
+
+// UnknownPayloadTypeError returns the opaque error value that is returned when
+// `jwt.Parse` fails due to not being able to deduce the format of
+// the incoming buffer
+func UnknownPayloadTypeError() error {
+	return errUnknownPayloadType
+}
+
 type parseError struct {
 	error
 }

jwt/jwt.go

@@ -6,7 +6,6 @@ package jwt
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"io"
 	"sync/atomic"
@@ -17,15 +16,6 @@ import (
 	"github.com/lestrrat-go/jwx/v3/jwt/internal/types"
 )
 
-var errInvalidJWT = errors.New(`invalid JWT`)
-
-// ErrInvalidJWT returns the opaque error value that is returned when
-// `jwt.Parse` fails due to not being able to deduce the format of
-// the incoming buffer
-func ErrInvalidJWT() error {
-	return errInvalidJWT
-}
-
 // Settings controls global settings that are specific to JWTs.
 func Settings(options ...GlobalOption) {
 	var flattenAudience bool
@@ -295,7 +285,7 @@ OUTER:
 
 			break OUTER
 		case jwx.InvalidFormat:
-			return nil, ErrInvalidJWT()
+			return nil, UnknownPayloadTypeError()
 		case jwx.UnknownFormat:
 			// "Unknown" may include invalid JWTs, for example, those who lack "aud"
 			// claim. We could be pedantic and reject these

jwt/jwt_test.go

@@ -1452,7 +1452,7 @@ func TestGH850(t *testing.T) {
 	var testToken = `eyJhbGciOiJFUzI1NiJ9.eyJzdWIiOiJ0ZXN0IiwiaWF0IjoxNjY2MDkxMzczLCJmb28iOiJiYXIifQ.3GWevx1z2_uCBB9Vj-D0rsT_CMsMeP9GP2rEqGDWpesoG8nHEjAXJOEQV1jOVkkCtTnS18JhcQdb7dW4i-zmqg.trailing-rubbish`
 
 	_, err := jwt.Parse([]byte(testToken), jwt.WithVerify(false))
-	require.True(t, errors.Is(err, jwt.ErrInvalidJWT()))
+	require.True(t, errors.Is(err, jwt.UnknownPayloadTypeError()))
 }
 
 func TestGH888(t *testing.T) {