Return type as generic base class is not allowed

[umurgdk]

I have a method which returns an object typed ParserThen(Array(Something), Array(Something)) and its parent class is Parser(Array(Something)) but compiler complains about its type. I simplified the problem.

https://carc.in/#/r/tut

Error in line 30: instantiating 'test()'

in line 23: type must be A(Int32), not C(Char, Int32)

class A(T)
    def val
        [] of T
    end
end

class B(T) < A(T)
end

class C(T,U) < A(U)
    def initialize(@first : A(T), @second : A(U))
    end

    def first
        @first.val
    end

    def val
        @second.val
    end
end

def test : A(Int32)
    a = A(Char).new
    b = B(Int32).new

    C.new a, b
end

test

[asterite]

Hi,

Please provide the full code that triggers the error you are getting. I'm getting:

Error in ./foo.cr:1: undefined constant Parser

There's nothing we can do with that to help you.

[umurgdk]

@asterite I just simplified problem on the description and include a carc.in link to it.

[bcardiff]

The generics class model have some pitfalls. Inheriting from a generic class I think is the problematic thing currently. Composition instead of inheritance might offer a workaround.

https://carc.in/#/r/tv9
using C.new(a, b) as A(Int32)
leads to can't cast to A(Int32) because it was never instantiated (Exception)

https://carc.in/#/r/tvc
forcing a foo = A(Int32).new
leads to a Error: you've found a bug in the Crystal compiler. Bug: trying to upcast A(Int32) <- C(Char, Int32)

[bcardiff]

Might be related to #846

[asterite]

Yes, inheriting from a generic class will probably won't work in many cases, so for now I'd advice not to use that "feature". We'll probably fix that once type annotations of instance variables are mandatory. Chances of fixing this before that point are very low.

[asterite]

Closed in favor of #2665