SLS specifies syntax of type parameter clause as
TypeParamClause ::= ‘[’ VariantTypeParam {‘,’ VariantTypeParam} ‘]’
FunTypeParamClause::= ‘[’ TypeParam {‘,’ TypeParam} ‘]’
VariantTypeParam ::= {Annotation} [‘+’ | ‘-’] TypeParam
TypeParam ::= (id | ‘_’) [TypeParamClause] [‘>:’ Type] [‘<:’ Type] {‘<%’ Type} {‘:’ Type} {‘<%’ Type} {‘<%’ Type}
where we see >:, <:, <%, <%, : as allowed reserved names in type parameter clause. Is there a way we could use generalised type constraint symbolic names <:<, =:= in the type parameter clause such that
def f[T =:= 42] = ???
would expand to
def f[T](implicit ev: T =:= 42) = ???
similar to how context bound
def f[T: Numeric] = ???
expands to
def f[T](implicit ev: Numeric[T]) = ???
In 2.13 (which supports singleton types if you are curious about constraining on singletons) you can do things like:
@ import $plugin.$ivy.`org.typelevel:kind-projector_2.13.1:0.11.0`
import $plugin.
@ type a = 23
defined type a
@ def f[N : * =:= a]: Unit = ()
defined function f
@ f[a]
@ f[23]
@ f[25]
cmd9.sc:1: Cannot prove that 25 =:= Int(23).
val res9 = f[25]
^
Compilation Failed
@ def g[N : * =:= 16]: Unit = ()
defined function g
@ g[16]
@ g[23]
cmd11.sc:1: Cannot prove that 23 =:= 16.
val res11 = g[23]
^
Compilation Failed
So, yes, it seems possible. You just have to use kind projectors to apply second parameter.
With <:< it should be the same story:
@ def h[N : * <:< 16]: Unit = ()
defined function h
@ h[16]
@ h[17]
cmd13.sc:1: Cannot prove that 17 <:< 16.
val res13 = h[17]
^
Compilation Failed