Bunched CanThrows

[odersky]

One problem with the saferException scheme is that so far each CanThrow capability has to be passed separately. It's not yet possible to bunch several exceptions in a single CanThrow capability.

It looks like we could use dependency injection scheme explained here and here to aggregate CanThrow. Using this scheme, a capability to enable throwing an IOException and an SQLExcecption would be

  CanThrow[(IoException, SQLException)]

The discussion on dependency injection shows why & would not work for aggregating exceptions (it would lead to given search ambiguities) but tupling is a good alternative.

[odersky]

This would have ramifications on the translation of throws clauses.

def f(): T throws E1 | E2

should now translate to

def f()(using CanThrow[(E1, E2)]): T

[odersky]

As noted by @hamzaremmal both this scheme and the current scheme of curried CanThrow context function types have the disadvantage that they don't support weakening the exception contract in subclasses. I.e. none of the two supports

class C1:
  def f: A throws E1 | E2
class C2 extends C1:
  override def f: A throws E1

It seems this needs some special treatment in the overriding checks.