Daemon crash when removing type parameter

[msullivan]

In certain circumstances the daemon will crash with an array out of bounds error when a type parameter is removed from a class:

[case testRemoveGeneric]
[file a.py]
from typing import Generic, TypeVar
T = TypeVar('T')
class A(Generic[T]): pass

[file a.py.2]
from typing import Generic, TypeVar
T = TypeVar('T')
class A: pass

[file b.py]
from a import A
from d import take

def foo() -> None:
    take([A()])

[file d.py]
from typing import List
from a import A
def take(x: List[A]): pass

[builtins fixtures/list.pyi]
[out]
==

crashes with a traceback ending in

  File "/home/msullivan/src/mypy/mypy/checkexpr.py", line 812, in check_callable_call
    callee, args, arg_kinds, formal_to_actual, context)
  File "/home/msullivan/src/mypy/mypy/checkexpr.py", line 1027, in infer_function_type_arguments
    strict=self.chk.in_checked_function())
  File "/home/msullivan/src/mypy/mypy/infer.py", line 31, in infer_function_type_arguments
    callee_type, arg_types, arg_kinds, formal_to_actual)
  File "/home/msullivan/src/mypy/mypy/constraints.py", line 63, in infer_constraints_for_callable
    c = infer_constraints(callee.arg_types[i], actual_type, SUPERTYPE_OF)
  File "/home/msullivan/src/mypy/mypy/constraints.py", line 140, in infer_constraints
    return template.accept(ConstraintBuilderVisitor(actual, direction))
  File "/home/msullivan/src/mypy/mypy/types.py", line 667, in accept
    return visitor.visit_instance(self)
  File "/home/msullivan/src/mypy/mypy/constraints.py", line 325, in visit_instance
    if tvars[j].variance != CONTRAVARIANT:

[ilevkivskyi]

Isn't it just a duplicate of #3279 but "reversed" (in the existing issue, the out of bound errors appear when a non-generic class is turned into generic)?

[ilevkivskyi]

Actually the original issue has a recipe for this "direction" too, see #3279 (comment) (although the crash actually happens in a different visitor I believe the cause is the same).

[msullivan]

Yeah I think so