Better definition of the traits an expression can have

• is it a literal ?
• is it statically evaluable ?
• can it initialize a variable ?
• can it be used as a const alias ?

is not well defined enough.

For example const alias rng = 0 .. 2, is currentry rejected but the RHS is accepted as argument of a generic parameter.

Also this is somewhat related to the fact that a type can be for a variable or if it's only internal (TypeRange).