Added tests for "default" generic method implementations to work like #:methods.

Specifically, it should be possible for a "default" implementation to have
unsupported methods, and to use define/generic.

pkgs/racket-pkgs/racket-test/tests/generic/errors.rkt

@@ -1,6 +1,6 @@
 #lang racket/base
 
-(require racket/generic unstable/macro-testing)
+(require racket/generic racket/engine unstable/macro-testing)
 
 (module+ test
   (require rackunit)
@@ -40,4 +40,16 @@
                                   #:methods gen:foobar
                                   [(define foo 0)])
                           'ignore))))
+  (check-exn #rx"foo: not implemented for 5"
+             (lambda () (let ()
+                          (define-generics foobar
+                            [foo foobar x]
+                            #:defaults ([number?]))
+                          ;; a failing implementation may loop forever,
+                          ;; due to self-reference to the generic method foo,
+                          ;; so using an engine here to limit the time.
+                          (define e (engine (lambda (suspend?) (foo 5 6))))
+                          ;; 1000 ms should be far more than enough.
+                          (or (engine-run 1000 e)
+                              (error "computation did not terminate")))))
   )

pkgs/racket-pkgs/racket-test/tests/generic/syntax-errors.rkt

@@ -72,7 +72,20 @@
      (define stream-first car)
      (define stream-rest cdr)
      (define stream-empty? null?)])
-   #:defined-table stream-table))
+   #:defined-table stream-table)
+
+ (define-generics stream
+   (stream-first stream)
+   (stream-rest stream)
+   (stream-empty? stream)
+   #:defaults
+   ([box?
+     (define/generic first stream-first)
+     (define/generic rest stream-rest)
+     (define/generic empty? stream-empty?)
+     (define stream-first (compose first unbox))
+     (define stream-rest (compose rest unbox))
+     (define stream-empty? (compose empty? unbox))])))
 
 (check-bad-syntax