types of constant default values of keyword arguments not derived?

[Nikodemus Siivola]

  (defun foo (&key (a :x))
    (declare (fixnum a))
    a)

does not cause a warning. (BTW: old SBCL issued a warning, but for a
function, which was never called!)

[Tobias C. Rittweiler (tcr)]

Fixing this bug will not cope well with DEFSTRUCT.

(defstruct foo (x (sb-int:missing-arg) :type fixnum))

will emit a warning because MAKE-FOO will be defined as follows

  (DEFUN MAKE-FOO (&KEY ((:X #:DUM971) (SB-INT:MISSING-ARG)))
    (DECLARE (TYPE FIXNUM #:DUM971))
    (SB-KERNEL::%MAKE-STRUCTURE-INSTANCE-MACRO
     #<SB-KERNEL:DEFSTRUCT-DESCRIPTION FOOBAR> '((:SLOT T . 1)) #:DUM971))

That kind of initialization-value checking is a useful violation of what CLHS specifies
about DEFSTRUCT. I recall some comment or something which mentions this
issue explicitly, perhaps by WHN, but I can't find it right now.

[Tobias C. Rittweiler (tcr)]

The last remark was bogus as SB-INT:MISSING-ARG is declared to have return-type NIL
which, of course, is a subtype of every other type.

Must be something wrong on how I tried to fix this bug.

[Stas Boukarev (stassats)]

33e6b37eaf92e41cbd9b7d951442d145cd653b3b