Cannot turn existing dictionary into TypedDict

I expected any of these forms to be allowed by MyPy, but they aren't:

# typed_dict_test.py
from typing import TypedDict

class D(TypedDict):
    a: str
    b: int


d_untyped = {"a": "foo", "b": 1}

d0 = D({"a": "foo", "b": 1})  # The only one that works
print(d0)

d1 = D(d_untyped)
print(d1)

d2 = D(**d_untyped)
print(d2)

d3 = D(dict(d_untyped))
print(d3)

d4 = D(dict(**d_untyped))
print(d4)

$ python typed_dict_test.py
{'a': 'foo', 'b': 1}
{'a': 'foo', 'b': 1}
{'a': 'foo', 'b': 1}
{'a': 'foo', 'b': 1}
{'a': 'foo', 'b': 1}
$ python -V
Python 3.8.3
$ mypy typed_dict_test.py 
typed_dict_test.py:13: error: Expected keyword arguments, {...}, or dict(...) in TypedDict constructor  [misc]
    d1 = D(d_untyped)
         ^
typed_dict_test.py:16: error: Expected keyword arguments, {...}, or dict(...) in TypedDict constructor  [misc]
    d2 = D(**d_untyped)
         ^
typed_dict_test.py:19: error: Expected keyword arguments, {...}, or dict(...) in TypedDict constructor  [misc]
    d3 = D(dict(d_untyped))
         ^
typed_dict_test.py:22: error: Expected keyword arguments, {...}, or dict(...) in TypedDict constructor  [misc]
    d4 = D(dict(**d_untyped))
         ^
Found 4 errors in 1 file (checked 1 source file)
$ cat mypy.ini 
[mypy]
warn_redundant_casts = True
warn_unused_configs = True
pretty = True
show_error_codes = True

disallow_any_generics = True
disallow_subclassing_any = True
disallow_untyped_calls = True
disallow_incomplete_defs = True
check_untyped_defs = True
disallow_untyped_decorators = True
no_implicit_optional = True
warn_unused_ignores = True
warn_return_any = True
no_implicit_reexport = True

I checked that cast(D, ...) works, but I wonder if this is the right way to proceed, or a workaround.

Start by reproducing the diagnostic with the TypedDict example in the issue and inspect mypy's TypedDict constructor checking. Compare the accepted literal and dict(...) forms with the rejected existing-dictionary forms; done means the intended supported conversions are type-checked consistently with runtime behavior and covered by tests.