Lint select from Unit, or Int that incurs widening

[som-snytt]

Fixes scala/bug#12728

[som-snytt]

Some current inconsistency.

scala> 42.ceil
val res0: Float = 42.0

scala> def i = 42
def i: Int

scala> i.ceil
       ^
       warning: Widening conversion from Int to Float is deprecated because it loses precision. Write `.toFloat` instead.
val res1: Float = 42.0

scala> def c = 'c'
def c: Char

scala> c.ceil
val res2: Float = 99.0

where c.ceil is pronounced "Cécile" and is even less likely to be intended.

With the new warning, the current warning is less correct.

[som-snytt]

It warns in two spots that specialize Array[Unit].

The other thought is that (as with the current warning), any promotion is dubious, it doesn't matter which member is selected.

Then just need definitions.RichFloatClass.

[som-snytt]

The check is only at typer, which gives a pass to

case Literal(Constant(())) =>

Not sure why it was deemed bad to allow case _: () => as the unit value ought to be a singleton. (It is not Unit with Singleton.) I guess it doesn't matter because the test is always boxed unit equality. There's no advantage to case _: Unit.

Something became ().asInstanceOf[Unit] in fields, a private lazy val after init. (That was the test execution will be non-parallel in PartestDefaults.)

[lrytz]

LGTM, and very useful.

[lrytz]

long.isNaN goes through float2Float(long.toFloat), not double2Double, so this seems to cover it all.

[som-snytt]

Rebased, and enhanced partest --realeasy to skip tests that run on today's JDK.

-- 1 - run/blank.scala                           [skipped on Java 19, compiling against JDK8 but must run on 11+]

It would be ideal if --realeasy did not skip in case of explicit scalac: --release:8 but that was too subtle for me.

The option also adds run/t6240-universe-code-gen.scala which is annoying to forget to check. (I had actually fixed it in the wrong commit.)

Selected 1810 tests drawn from 1 named test categories, 2 tests modified on this branch, and other tests you might have forgotten

In case of selection failure,

sbt:root> partest junk
Discarding 1 invalid test paths

Selected 0 tests drawn from the well of despair. I see you're not in a testing mood.
[info] Passed: Total 0, Failed 0, Errors 0, Passed 0

[lrytz]

Thank you!