Fix narrowing unions in tuple pattern matches

[edpaget]

Allows sequence pattern matching of tuple types with union members to use an unmatched pattern to narrow the possible type of union member to the type that was not matched in the sequence pattern.

For example given a type of tuple[int, int | None] a pattern match like:

match tuple_type:
    case a, None:
         return
    case t:
         reveal_type(t) # narrows tuple type to tuple[int, int]
         return

The case ..., None sequence pattern match can now narrow the type in further match statements to rule out the None side of the union.

This is implemented by moving the original implementation of tuple narrowing in sequence pattern matching since its functionality should be to the special case where a tuple only has length of one. This implementation does not hold for tuples of length greater than one since it does not account all combinations of alternative types.

This replaces that implementation with a new one that builds the rest type by iterating over the potential rest type members preserving narrowed types if they are available and replacing any uninhabited types with the original type of tuple member since these matches are only exhaustive if all members of the tuple are matched.

Fixes #14731

[github-actions[bot]]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[edpaget]

Hi all, anyone I could work with to get this reviewed?