It's a big patch, but the summary is that it's okay to use a pattern in
an erased position if either:
- the pattern can also be solved by unification (this is the same as
'dot patterns' for matching on non-constructor forms)
- the argument position is detaggable w.r.t. non-erased arguments, which
means we can tell which pattern it is without pattern matching
The second case, in particular, means we can still pattern match on
proof terms which turn out to be irrelevant, especially Refl.
Fixes#178
Allow matching rather than unification, as long as it doesn't solve any
metavariables on the way. I noticed a potential unification bug on the
way, forgetting to update whether holes are solved when unifying
argument lists.
Like Idris 1, these are implicitly added on encountering a repeated name
or a non-constructor application. Unlike Idris 1 (and Blodwen) they are
checking by unification rather than matching (which means in particular
that function argument names can't be bound in dot patterns) which is
slightly less expressive, but better overall because matching is
potentially more error prone.
