Disable old context function syntax

[odersky]

Fixes #10697

[bishabosha]

these changes will allow to use the context arrow to define a no-arg lambda
val f = () ?=> 0 which is typed as a plain Function0 () => Int

[odersky]

@bishabosha Can you take the PR over please and fix the problem? I am swamped, so won't have the time. Thanks!

[bishabosha]

this should be ready to go

[nicolasstucki]

@bishabosha there is still a failure

-- Error: tests/pos/i7778.scala:11:34 ----------------------------------------------------------------------------------
  final case class Foo[A, B](run: () ?=> Int)
                                  ^^
                                  context function types require at least one parameter

[bishabosha]

@bishabosha there is still a failure

oh right, i did not expect to see that explicitly as a pos test, seems confusing to want to allow 0 arg ctx fn. Ill move it to neg

[nicolasstucki]

It looks like we also need to fix the community build https://github.com/lampepfl/dotty/runs/1550414420

[bishabosha]

It looks like we also need to fix the community build https://github.com/lampepfl/dotty/runs/1550414420

I think it will be ok, both tasks passed after the compiler sources were fixed in 59bca1c

[bishabosha]

only community build to go - hopefully it passes