'TypeVar's `bound` parameter does not play nice with overloads(?)

Bug Report
'TypeVar's bound parameter does not play nice with overloads(?)

To Reproduce

from typing import Union, TypeVar, overload

class C1:...
class C2:...

T = TypeVar('T', bound=Union[C1, C2])

@overload
def update(elt: C1) -> C1:...

@overload
def update(elt:C2) -> C2:...

def update(elt: Union[C1, C2]) -> Union[C1, C2]:
  pass

def get_copy(elt: T) -> T:
  new_elt = update(elt)
  return new_elt

Expected Behavior

This should type-check(?)

Actual Behavior

I get errors:

type_tests.py:47: error: Argument 1 to "update" has incompatible type "T"; expected "B1"
type_tests.py:48: error: Incompatible return value type (got "D1", expected "T")    

If I replace T = TypeVar('T', bound=Union[C1, C2]) with T = TypeVar('T', C1, C2, Union[C1, C2]), it
passes just fine

Your Environment

• Mypy version used: mypy 0.812
• Mypy command-line flags: None: command was mypy type_tests.py
• Mypy configuration options from mypy.ini (and other config files): default
• Python version used: 3.7.8
• Operating system and version: Ubuntu 16.04

Reproduce the reported errors by running mypy type_tests.py with the TypeVar bound and overload definitions shown in the issue, then compare them with the constrained-TypeVar variant. Trace how mypy handles the bound during overload resolution and return-type checking; done means determining the intended behavior and adding a regression test or documenting why the bound is rejected.