This is to record the red->green change in behavior between K1 and K2,
when K2 correctly handles resolution/inference of an extension function
with definitely non-nullable type.
Previously, it was led to plainly adding NullableType <: T constraint
which silently led to successful call completion.
What is suggested is just marking such initial constraint
as unsuccessful.
In K1, the error was reported just via additional type checking
mechanism being run after call completion.
^KT-58665 Fixed
That issue might be fixed via changing
TypeVariableMarker.shouldBeFlexible at ConeConstraintSystemUtilContext
but this and some other tricks have been added because of incorrect
handling of constraints where type variable has a flexible bound
^KT-51168 Fixed
1. this should've been only done if the language feature for validating
that is disabled;
2. that feature probably won't matter by the time FIR is stable;
3. it only worked because type enhancement of type arguments is broken
anyway - a more correct hack would be to provide a custom
ConeTypePreparator.
marat.akhin