We should not rely on this feature. It can be quite expensive.
We invoke is_convertible in several places, in particular, if we are using overloading. For example, the frontend uses is_convertible to check which overload should be used. Thus, it will make several calls such as
is_convertible(num, Nat)
If is_convertible starts unfolding opaque definitions, we would keep expanding num.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
