Coersion to Unit from error type leads to misleading type mismatches:
"expected <expected lambda return type> found Unit", despite no user-provided Unit / empty lambda.
These diagnostics were collected, but not reported before, and that had been disguising the issue for a while.
KT-34729 Fixed
Expression will be checked against expected type later.
Theoretically, this is not very good, but it aligns with the old
inference, plus it helps avoiding multiple type mismatch diagnostics.
Consider following case:
fun foo(): Unit = run { "hello" }
Previously, NI would analyze lambda body without expected type, because
it is a type variable 'R' from 'run', which hasn't been fixed yet. This
leads to treating "hello" as lambda-return argument and adding bogus
'String' constraint on 'R', and, consequently, type mismatch.
Now, we peek into current constraint system and check if return-type of
lambda is type variable with upper-Unit constraint (which is exactly
condition for its body to be Unit-coerced). If so, then we provide
expected Unit-type for body explicitly, and the rest will be done
automatically (in particular, in aforementioned example "hello" wouldn't
be treated as lambda return argument).
simon.ogorodnik