Generalized constraints within type parameter clause?

Question

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]) = ???

Answer 1

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