Replace delegate match with summonFrom.

[odersky]

Instead of having a separate syntactic construct (latest incarnation: delegate match), provide
a special method summonFrom in scala.compiletime.

delegate match { 
  case ev_1: TC_1 => e_1
  ...
  case ev_n: TC_n => e_n
}

becomes

summonFrom { 
  case given ev_1: TC_1 => e_1
  ...
  case given ev_n: TC_n => e_n
}

summonFrom is currently implemented by compiler magic. It could be implemented as a macro if we introduce an additional way to control when inline methods expand. See the comment in Applications.scala.

Based on #7194.

[milessabin]

I'll update the shapeless-3 branch in the community build as soon as there's a nightly with these changes.

[odersky]

@milessabin How do we proceed with the bootstrap? Here's one way I see would work, but maybe there's something simpler:

1. I roll everything up to 2b8ba68 into Allow given bindings in patterns #7194 and we merge that first. That gives us summonFrom as well as delegate match.
2. You update shapeless in the CB once (1) is in the nightly.
3. We merge this PR once (2) is merged.

Should we do it like this?

[milessabin]

Should we do it like this?

Or simpler ... just let shapeless break in the community build and I'll fix it up afterwards.

[odersky]

I was told by @nicolasstucki we can't get a nightly anymore if the CB breaks. So we'll do it in steps.

[odersky]

@milessabin I was puzzled why the community build passed, even though delegate match is gone. It seems shapeless still uses implicit match, right? That will be the next to go but in a different PR. So we can get this one in if I get a review thumbs up and then it's over to you for the shapeless update.

[odersky]

@biboudis This is actually a stack for 3 PR's:

#7150: Reference docs only, changes the given syntax
#7194: Allow given in patterns
#7201: This PR

I believe they are best reviewed in order, but it's easiest if we do fixes and merges just on this PR.

[milessabin]

It seems shapeless still uses implicit match

Oh! Yes ... I went straight from implicit to given and never changed the matches over.

Yup, go ahead and merge this. Do you want to do the separate PR that removes implicit match right away, or will there be a new nightly including this PR before then?

[odersky]

Yup, go ahead and merge this. Do you want to do the separate PR that removes implicit match right away, or will there be a new nightly including this PR before then?

We'll do a new nightly before.