from collections.abc import Sequence

class Foo: ...

def concat[L: Foo, R: Foo](
    left: L | Sequence[L],
    right: R | Sequence[R], /
) -> list[L | R]:
    match left, right:
        case Foo(), Foo():
            return list( (left, right) )  # ❌
        case Foo(), [*rvalues]:
            return list( (left, *rvalues) )  # ❌
        case [*lvalues], Foo():
            return list( (*lvalues, right) )  # ❌
        case [*lvalues], [*rvalues]:
            return list( (*lvalues, *rvalues) )
        case _:
            raise TypeError
            
def concat2[L: Foo, R: Foo](
    left: L | Sequence[L],
    right: R | Sequence[R], /
) -> list[L | R]:
    if isinstance(left, Foo) and isinstance(right, Foo):
        return list( (left, right) )  # ❌
    elif isinstance(left, Foo) and isinstance(right, Sequence):
        return list( (left, *right) )  # ❌
    elif isinstance(left, Sequence) and isinstance(right, Foo):
        return list( (*left, right) )  # ❌
    elif isinstance(left, Sequence) and isinstance(right, Sequence):
        return list( (*left, *right) )
    else:
        raise TypeError