Deprecation story for auto-tupling

[adriaanm]

(to flesh out)

[eed3si9n]

Ref:

• Drop -Y flags that change language semantics Drop -Y flags that change language semantics scala#6505
• Spec doesn't mention automatic tupling Auto-tupling isn't spec'ed and should perhaps be deprecated (not just linted) bug#3583
• Let's drop auto-tupling
• Remove Two Warts: Auto-Tupling and Multi-Parameter Infix Operations Remove Two Warts: Auto-Tupling and Multi-Parameter Infix Operations scala3#4311

[SethTisue]

@adriaanm is this something we need to make sure happens for RC1?

[adriaanm]

I think we can figure this (and any other deprecation strategy) out early in the 2.13.x cycle.

[Jasper-M]

I would just like to point out that once you have HList-like tuples in Scala 3, having auto-tupling only for arguments which have a Tuple upper bound is a pretty great way to have syntax-free arity abstraction. It seems a bit strange to on the one hand build arity abstraction into the language and std library, and on the other remove a feature that could make arity abstraction great, which was available when arity abstraction was hard to do anyway.

I don't think it could still cause confusion or weird runtime errors either, since auto-tupling will only happen when a tuple is already expected.

The reason that right now not much breaks between removing auto-tupling for Any or Unit bounded arguments, and removing it for everything, is probably that

• currently you have to use a bunch of Shapeless macros to convert between tuples and hlists,
• currently tuples don't even share a common type, other than Product which all case classes have, and
• no one in their right mind would write def foo(a: (Int, Int)) or def foo[A <: (Int, Int)](a: A), cause right now it's used exactly the same as def foo(a: Int, b: Int) except it has a bunch of extra overhead.

Without auto-tupling I think I would actually prefer (syntax-wise) shapeless hlists over built-in tuples-as-hlists. To me foo((1, 2, 3)) looks worse/weirder/uglier than foo(1 :: 2 :: 3 :: HNil).

[SethTisue]

new plan at scala/scala3#4311 (comment)