[Python] type checking: assignment of merged dicts not compatible, even though individual...

I have a situation where I want to assign the union of two dicts (using |) to a typed dict variable. Each of the dicts on its own is assignment compatible, but assigning the result of the merge where at least one of the dicts is a literal results in MyPy and Pyright both reporting an error:

from typing import Literal

type MyType = Literal['a', 'b']


def f() -> dict[MyType, str]:
d1: dict[MyType, str] = {'a': 'foo'} # <-- no error
d2: dict[MyType, str] = {'b': 'bar'} # <-- no error
d3: dict[MyType, str] = d1 | d2 # <-- no error
d4: dict[MyType, str] = {'a': 'foo'} | {'b': 'bar'} # <-- error "dict[str, str]" is not assignable to "dict[MyType, str]"
d5: dict[MyType, str] = {'a': 'foo'} | d2 # <-- error "dict[str, str]" is not assignable to "dict[MyType, str]"

return d4


If type checkers can recognize that each of the dicts on its own is assignment compatible, why can they not recognize that the merge/union of two compatible dicts is assignment compatible?

Update

If I cast the keys as MyType, MyPy is happy, but Pyright still complains: i.e., {cast(MyType, 'a'): 'foo'} | {cast(MyType, 'b'): 'bar'} works for MyPy. And if I cast the whole union, that works for both: i.e. cast(dict[MyType, str], {'a': 'foo'} | {'b': 'bar'}). Seems like they should be able to figure it out, even without the cast, though.

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