##sys#foreign-pointer-argument is declared to accept a pointer only, but is possibly invoked with a boolean

[sjamaan]

When you use let-location or define-foreign-variable and declare a pointer type, you can set it to a NULL value by setting it to #f in Scheme and this works correctly. However, the scrutinizer thinks that's bad:

$ cat test.scm
(define-foreign-variable bar c-pointer)

(set! bar #f)

(let-location ((x c-pointer #f))
  ((foreign-lambda void "printf" c-string c-pointer) "%p\n" x))
$ csc -scrutinize test.scm
Warning: at toplevel:
  (test.scm:3) in procedure call to `##sys#foreign-pointer-argument', expected argument #1 of type `pointer', but was given an argument of type `boolean'

Warning: at toplevel:
  (test.scm:5) in procedure call to `##sys#foreign-pointer-argument', expected argument #1 of type `pointer', but was given an argument of type `boolean'

The underlying problem is that ##sys#foreign-pointer-argument is declared as accepting only pointer types (which is correct, it will barf otherwise), but the initializer/set value is #f, a boolean which is allowed as a value representing a NULL pointer. The if check in library.scm:1077 takes care of this situation, but the scrutinizer is unable to verify that the boolean isn't #t when it passes the if and takes the ##sys#foreign-pointer-argument branch (and it might be!). This output be seen more clearly when calling "csc -debug 2 test.scm".

We could do a few things here:

1) Add a ##core#the assertion that states the argument is definitely a pointer. That kind of kills the scrutinizer's effectiveness at warning, moving the error to runtime (it's also incorrect in case anything else is passed).
2) Map #t to #f (a NULL pointer eventually in the C translation). That's inconsistent with the rest of core.
3) Map #t to an error. This still moves the error to a runtime one but keeps the scrutinizer warnings for all non-boolean types.
4) Keep it the way it is. That will give bogus warnings on this kind of code.
5) Remove ##sys#foreign-pointer-argument from types.db or change it to accept a boolean. The former again gets rid of the advantage of having the scrutinizer and the latter is blatantly incorrect.
6) Change the ##sys#foreign-pointer-argument to accept booleans and only return C_SCHEME_FALSE and barf on C_SCHEME_TRUE.
7) Augment the type system with yet another special case for #f, adding a "false" type that is a subtype of "boolean".

I don't know which option I like most (or dislike least). 7 is definitely the most "thorough" option, similar to the special handling of null, pair and list(-of) types, but it raises the complexity of the scrutinizer another notch. Other acceptable solutions would probably either be 3 or 6, which both basically come down to the same thing. I think whatever we do, we should probably postpone it until after the next release unless someone comes up with a clever, noninvasive alternative of course :)

[sjamaan]

Fixed by ddb2b635013492c660b017b1cc29118e9d73165d