Defining abstract methods with generics fails

[spinscale]

I have an issue with abstract methods and generics (note, java developer speaking here, so this might be just an misunderstanding).

When defining the following abstract class

abstract class Foo(T)
  abstract def set(copy : Array(T)) 
end

I expected the implementation to be

class Bar < Foo(String)
  def set(copy : Array(String))
  end
end

This however gives a compile time error, where as using def set(copy : Array) works.

[vladfaust]

Looks like a bug for me, because this code raises no error:

class Foo(T)
  def foo(x : T)
    {% raise NotImplementedError %}
  end
end

class Bar < Foo(String)
  def foo(x : String)
    puts x
  end
end

Bar.new.foo("foo")

[RX14]

Definitely a bug, workaround is just to loosen the type restriction.

[vladfaust]

Got this bug again while creating an abstract rescue HTTP handler (https://carc.in/#/r/5tcy):

abstract class Rescuer(T)
  def call(context)
    call_next(context)
  rescue ex : T
    handle(ex)
  end

  abstract def handle(ex : T)
end

class BasicRescuer < Rescuer(Exception)
  def handle(ex : Exception)
    puts ex.inspect_with_backtrace
  end
end

BasicRescuer.new

Error in line 8: abstract `def Rescuer(T)#handle(ex : T)` must be implemented by BasicRescuer

I post it to prove that there are some real-world use-cases.

[asterite]

Workaround: don't define the abstract method at all.

[vladfaust]

As mentioned above in #6762 (comment), defining a non-abstract method with T and then overriding with Type works (https://carc.in/#/r/5tex). But it doesn' work in this particular case (https://carc.in/#/r/5tey):

The example could be simplified, sorry about that

require "http/server"

abstract class Rescuer(T)
  include HTTP::Handler

  def call(context)
    call_next(context)
  rescue error : T
    handle(error)
  end

  def handle(error : T)
    {% raise "Must be implemented" %}
  end
end

class BasicRescuer < Rescuer(Exception)
  def handle(error : Exception)
    puts error.inspect_with_backtrace
  end
end

server = HTTP::Server.new([BasicRescuer.new]) do |context|
  context.response.print("Hello\n")
end

server.bind_tcp(5000)
puts "Listening"
server.listen

in line 9: Must be implemented

As @asterite mentioned in #7200, there might be a problem with module inclusion.

It also seems to be related to #6701.

[asterite]

Just to clarify, the abstract method check doesn't work well with generic inheritance.

Please try to avoid inheriting from generic types right now.