Generic inference restrictions

The types var T, out T and typedesc[T] cannot be inferred in a generic instantiation. The following is not allowed:

proc g[T](f: proc(x: T); x: T) =
  f(x)
proc c(y: int) = echo y
proc v(y: var int) =
  y += 100
var i: int
# allowed: infers 'T' to be of type 'int'
g(c, 42)
# not valid: 'T' is not inferred to be of type 'var int'
g(v, i)
# also not allowed: explicit instantiation via 'var int'
g[var int](v, i)