punning doesn't work in presence of :~ or ::

[sorawee]

This is OK:

#lang rhombus

fun foo(~x):
  x

This is not OK:

#lang rhombus

fun foo(~x :: Number):
  x

which produces the error:

#%literal: not an allowed literal term in: ~x

[usaoc]

I see why one would want to do this, but I’m not sure adding yet more special cases is the best idea—function headers already have a rather sugary grammar as is.