Given overridden descriptors D = d[i].
1. Find D*, subset of D:
returnType(d* from D*) <: returnType(d) for each d from D.
Always prefer var to val.
2. Prefer non-flexible return type to flexible.
Check for var/val overrides properly
(NB: this will report PROPERTY_TYPE_MISMATCH_ON_OVERRIDE
for all properties, not just overrides involving vars as it was before).
- Tests.
- No need for a separate diagnostic message regarding
return/property type conflict on override by delegation:
it is always a conflict of inherited signatures.
Overridden signatures should have compatible return types
(equal types for 'var').
Only relevant overrides should be taken into account.
Refactor inherited signatures check,
introduce a strategy interface for problem reporting.
Dmitry Petrov