Better error message for non-existent generics groups.

Closes PR13671.
(cherry picked from commit aac22e1ce5)

collects/racket/private/define-struct.rkt

@@ -319,7 +319,8 @@
             (raise-syntax-error #f
                                 "not a name for a generics group"
                                 gen:foo gen:foo))
-          (unless (identifier? gen:foo) (bad-generics))
+          (unless (and (identifier? gen:foo) (identifier-binding gen:foo))
+            (bad-generics))
           (define gen:foo-val (syntax-local-value gen:foo))
           (unless (and (list? gen:foo-val)
                        (>= (length gen:foo-val) 1))

collects/tests/generic/errors.rkt

@@ -0,0 +1,14 @@
+#lang racket/base
+
+(require racket/generic unstable/macro-testing)
+
+(module+ test
+  (require rackunit)
+
+  (check-exn #rx"not a name for a generics group"
+             (lambda () (convert-compile-time-error
+                         (struct foo () #:methods 3))))
+  (check-exn #rx"not a name for a generics group"
+             (lambda () (convert-compile-time-error
+                         (struct foo () #:methods bad))))
+  )

collects/tests/generic/tests.rkt

@@ -9,6 +9,7 @@
          (submod "equal+hash.rkt" test)
          (submod "custom-write.rkt" test)
          (submod "defaults.rkt" test)
+         (submod "errors.rkt" test)
          "syntax-errors.rkt"
          "base-interfaces.rkt"
          "contract.rkt"