mypy error: Cannot infer type argument 1 of "sum"

When using sum() to concatenate lists, mypy fails to infer the type argument of "sum", even if types are declared explicitly.

sum([[1],[2],[3]], []

or

foo: List[List[int]] = [[1],[2],[3]]
sum(foo, [])

both give:

error: Cannot infer type argument 1 of "sum"

This looks like a duplicate of #230. Here are two workarounds:

You can use a local variable to hold the empty list and annotate the variable (empty: List[int] = []) and pass the variable as the second argument to sum.
You can cast the empty list to the desired type: sum(foo, cast(List[int], []))

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