BUG: cast from Nil to Crystal::GenericInstanceType+

[ysbaddaden]

The following crashes Crystal 0.30.0, when it should report missing abstract methods. A workaround is to implement the abstract methods.

struct Foo
  include Indexable(Foo)
end

cast from Nil to Crystal::GenericInstanceType+ failed, at src/compiler/crystal/semantic/abstract_def_checker.cr:215:5:215 (TypeCastError)
  from src/compiler/crystal/semantic/abstract_def_checker.cr:0:5 in 'find_base_generic_instantiation'
  from src/compiler/crystal/semantic/abstract_def_checker.cr:129:7 in 'implements?'
  from src/compiler/crystal/semantic/abstract_def_checker.cr:104:12 in 'implements?'
  from src/compiler/crystal/semantic/abstract_def_checker.cr:95:7 in 'implements_with_ancestors?'
  from src/compiler/crystal/semantic/abstract_def_checker.cr:78:15 in 'check_implemented_in_subtypes'
  from src/compiler/crystal/semantic/abstract_def_checker.cr:64:5 in 'check_implemented_in_subtypes'
  from src/compiler/crystal/semantic/abstract_def_checker.cr:54:13 in 'check_single'
  from src/compiler/crystal/semantic/abstract_def_checker.cr:38:7 in 'check_types'
  from src/compiler/crystal/semantic/abstract_def_checker.cr:33:5 in 'run'