Opportunity for better precedence in given declarations with experimental.modularity

[propensive]

Compiler version

Latest main, 3393f7ea7704eb1dd553c96a805906821c61db5f.

Minimized example

I've found a couple of cases using the new "modularity" syntax where parentheses are required, but where the parser ought to be able to parse unambiguously without.

Assuming a typeclass definition such as,

trait Describable:
  type Self
  def describe(): String

We cannot write:

given 8 is Describable = () => "eight"

We must write,

given (8 is Describable) = () => "eight"

and likewise for String singleton literal types.

And with language.experimental.captureChecking, the parentheses are also required for:

given (using ord: Ordering[Int]^) => (Int is Describable^{ord}) = () => "integer"

This is just mild friction, though the error messages are not informative. Another related case where parentheses become necessary occurs in pattern matching (typically only in an inline context, because usually erasure obviates matching on the Self parameter):

An "old-style" typeclass could be matched with,

compiletime.summonFrom:
  case describable: Describable[T] => // ...

but the equivalent in "new style" must now be written,

compiletime.summonFrom:
  case describable: (T is Describable) => // ...