@kha We can now solve unification constraints of the form
?m unit =?= itactic
I'm not very confident this new extension will improve usuability
instead of creating new counter-intuitive behavior.
At least, in the process of implementing it, I fixed two bugs,
and removed a nasty hack that was being used in the unifier.
Thus, even if we disable this feature in the future, some good came out
of it.
Although, the new extension locally increases the number of constraints
that can be solved, it may prevent terms that could be elaborated before
from being elaborated. I am not too concerned at this point because
I could not construct a natural example. I encountered one case, but it
was due to a problem that I fixed in the previous commit.
I reconstruct it here for the record. Suppose we have a constraint
?m_1 ?m_2 =?= itactic
Without the fix from the previous commit, `itactic` would unfold too
`id_rhs Type (tactic unit)`, and the constrain would be solved as
?m_1 := (id_rhs Type)
?m_2 := (tactic unit)
It succeeds locally, but the elaboration fails later when it tries to
synthesize the type class `has_bind (id_rhs Type)`.
The previous fixes the problem by making sure `itactic` is unfolded to
`tactic unit` as expected. `id_rhs` is an auxiliary definition
used to implement smart unfolding. That being said, the user could in
principle define `itactic` as `@id Type (tactic unit)`, and the constraint
?m_1 ?m_2 =?= itactic
will be solved as
?m_1 := (@id Type)
?m_2 := (tactic unit)
which is not the solution we want.
e6a98ffe9c