Trailing commas for tuples

[letalvoj]

Trailing comma support is incomplete. Python allows statements such as

()  # reserved for `Unit` in scala
(1,)

The scala/scala3#9049 changes Tuple instantiation, to

Tuple()  // empty
Tuple(1) // size one
(2,3)

It would be nice if it was allowed to write

()    == Tuple()
(1, ) == Tuple(1)

instead. That would require to force () to be Unit with -scala2compat flag OR/AND migrate () to {} in cross-compiled codebases.

If that's not possible I'd vote for.

(, ) == Tuple()

CCing @nicolasstucki who is implementing the current changes.

[nicolasstucki]

The unit () syntax has been in Scala for too long to be able to remove it now without causing too much confusion. The trailing comma provides a good unambiguous shorthand.