It should not forbit it but allow it. Please notice that generics are almost
replaceable by () operator (it should be replaceable by [] operator but then it
only works when called explicitly, not by operator syntax):
template Sth(t: typedesc = int): typedesc =
type `Sth t` = object
value: t
`Sth t`
var x: Sth(int)
var y: Sth(float)
var z: Sth()
echo x
echo y
echo z
Which is a beautiful example for the fact that higher order types are just
endofunctors on types. And also, that's how generic data structures are built
in Zig.
Anyway, the analogue should also be valid:
type Sth[T = int] = object
value: T
var x: Sth[int]
var y: Sth[float]
var z: Sth[] # should be valid
echo x
echo y
echo z
But sadly, it's not.
Btw. actually, the template-based approach is much more powerful as it can
provide things that have no analogue in Nim's types:
template Sth(t: typedesc = int): typedesc =
when t is int: int
elif t is float: float
else: {.error: "Only ints or floats, please".}; void
var x: Sth(int) # x is int
var y: Sth(float)
echo x
echo y
when allows to customize concrete types but only for objects. Template-based
approach allows any type.
