Unhandled error: Missing method definition(s).

[YarinHeffes]

When defining an instance of a type-class, omitting method definitions results in an error that is not explicit about the absence of the method definition(s).

The following block,

(coalton-toplevel 
  (define-class (C :T)
    (m (:T -> Unit)))
  (define-instance (C Integer)))

results in the following error,

The value
  COMMON-LISP:NIL
is not of type
  COALTON-IMPL/PARSER/TOPLEVEL:INSTANCE-METHOD-DEFINITION

Suggested error message:

Instance definition for type-class C is missing definition(s) for method(s) M

[stylewarning]

Great find. A PR should test for both missing methods and extra methods.