Tuples created through *: do not have a Mirror.Of[T]

[juanjovazquez]

Compiler version

3.1.1-RC1
3.1.0

Minimized code

summon[Mirror.Of[Int *: String *: Boolean *: EmptyTuple]]

Output

no implicit argument of type deriving.Mirror.Of[Int *: String *: Boolean *: EmptyTuple] was found for parameter x of method summon in object Predef

Expectation

Output should be the same as:

summon[Mirror.Of[(Int, String, Boolean)]]

since

summon[(Int, String, Boolean) =:= (Int *: String *: Boolean *: EmptyTuple)]

The cause seems related to the fact that *: is an abstract class and those are excluded from being generic products. Relevant links:
https://github.com/lampepfl/dotty/blob/7a473349b3fb64e7c6c9dfcfb02758f4a2e3756c/library/src/scala/Tuple.scala#L318

https://github.com/lampepfl/dotty/blob/7a473349b3fb64e7c6c9dfcfb02758f4a2e3756c/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala#L233

https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/transform/SymUtils.scala#L77-L88

[odersky]

Isn't that a duplicate of some other issues? I could swear I have seen that discussed recently.

[bishabosha]

This seems related to #11008 maybe (not the same underlying issue, just whether we consider generic tuple types a Product)

[joroKr21]

What should be the element names? _n beyond 22? Just realised we can't call _n on an inductive tuple.

[nicolasstucki]

Beyond 22 we do not use _ names. There we use the apply method to access the elements by index (0 based in that casse).

[joroKr21]

Right but if we implement Mirror for *: tuples we need to provide something as the field names, otherwise it won't work. And we can't call _n on *: below 22 either.

[nicolasstucki]

Then we probably are missing a generalization/extension for mirrors that can handle generic tuples.

[armanbilge]

Not sure if it deserves its own issue, but seems that EmptyTuple itself doesn't have a Mirror.

[bishabosha]

Not sure if it deserves its own issue, but seems that EmptyTuple itself doesn't have a Mirror.

seems that EmptyTuple was never made a case object, so that's why (@nicolasstucki do you remember why?) - also neither is *: a case class, I would guess that both are not because they are meant to substitute for tuples of some sort.

[nicolasstucki]

There was no real reason why we did not make EmptyTuple a case object. Maybe we followed the design of the *:.

The *: cannot be a case class because it is not a real runtime class. The TupleN classes from Scala 2 are used and a new TupleXXL for larger cases (above 22).

I'm trying to make EmptyTuple a case object in #14972 to see if we can.

[bishabosha]

Right but if we implement Mirror for *: tuples we need to provide something as the field names, otherwise it won't work. And we can't call _n on *: below 22 either.

there is no requirement from the definitions of Mirror.SumOf or Mirror.ProductOf that Mirrors have the MirroredElemLabels type member, so we could avoid adding that refinement for generic tuples?
Edit: sorry it is in the Mirror trait definiton