#316 :default-initargs not respected in the MOP

[Pascal Costanza]

The following code triggers an incorrect error:

(use-package :clos)

(defclass my-generic-function (standard-generic-function)
()
(:metaclass funcallable-standard-class)
(:default-initargs
:method-combination (find-method-combination
(class-prototype (find-class 'my-generic-function))
'+ '())))

(defgeneric test (x)
(:generic-function-class my-generic-function))

(defmethod test + (x) 0)

The error message is this:

*** - STANDARD method combination, used by #<MY-GENERIC-
FUNCTION TEST>, allows
no method qualifiers except (:BEFORE :AFTER :AROUND):
#<STANDARD-METHOD + (#<BUILT-IN-CLASS T>)>

However, it should have accepted that method because the method
combination is specified to be '+.

[Bruno Haible]

Logged In: YES
user_id=5923

It's not a bug. See the MOP spec p. 19 / 20, description
of DEFGENERIC.
While for
:argument-precedence-order
:documentation
:generic-function-class
:method-class
:declare
the spec says that if the DEFGENERIC form doesn't contain
a specification for this attribute, this keyword does not occur
in the argument list for ENSURE-GENERIC-FUNCTION, for
:method-combination it just says
"The handling of the :method-combination option is
not specified."

Which means, the :method-combination may always be
provided by the DEFGENERIC macro, or never, or
sometimes.

So, trying to specify the method-combination through
:default-initargs is not portable.

But since there is no other apparent way to specify a
default method-combination for a generic-function class
- adding a method to ENSURE-GENERIC-FUNCTION or
to FIND-METHOD-COMBINATION will not help -
I implement this as a new feature in clisp.