Variadic generics parametrized by value

[dmitriy-serdyuk]

Crystal allows to parametrize generics by integer:

class A(T) end
puts A(2).new()

#<A(2):0x10fb4dff0>

Unfortunately, this doesn't work with variadic generics:

class A(*T) end
puts A(2).new()

Error in tst.cr:2: argument to Tuple must be a type, not 2

puts A(2).new()
     ^

Related to #4337

[asterite]

Use case? I think it's fine that it doesn't compile.

[dmitriy-serdyuk]

It could be useful for statically typed n-dimensional arrays when you don't know n in advance.

Imagine matrix multiplication operator that accepts only right inputs:

def dot(first : Matrix(N, M), second : Matrix(M, K)) : Matrix(N, K)
    ....
end

This wouldn't work for n-dimensional case.