Flesh Out Match Types

[odersky]

The docs on match types contains several aspects that are not implemented yet. In particular:

• The "cannot possibly match rule". See Opaque types - selftype encoding #5300 (comment) for an interaction with opaque types.
• Detection of overlapping patterns in general
• Typing rules for match terms that allow a match term to have a match type without having to cast.

[smarter]

@OlivierBlanvillain can thsi be closed ?