[red-knot] Infer precise types for len() calls #14598
Description
Sometimes, it is useful to know the precise length of a type:
reveal_type(len((0, 1, 2))) # revealed: Literal[3]This might help with narrowing:
a: tuple[Literal[1], int] | tuple[Literal[2], str, str]
if len(a) == 2:
reveal_type(a[0]) # revealed: Literal[1]Current behaviour of Mypy and Pyright:
def f(a: tuple[Literal[1], int] | tuple[Literal[2], str, str]) -> None:
if len(a) == 2:
reveal_type(a[0]) # mypy => Literal[1]
# pyright => Literal[1]
l1 = len(a)
reveal_type(l1) # mypy => int
# pyright => int
if (l2 := len(a)) == 2:
reveal_type(l2) # mypy => int
# pyright => Literal[2]
reveal_type(a[0]) # mypy => Literal[1]
# pyright => Literal[1, 2]