Refine FlexibleType nullification rules

[noti0na1]

Based on #23911

Fixes #23908

Could fix #23933, #23935, #23936, and #23937 (I tested locally)?

DO NOT MERGE: the explicit nulls tests are currently broken

cc @olhotak @HarrisL2

[olhotak]

Otherwise LGTM

[noti0na1]

Would you prefer this approach? @olhotak

[olhotak]

We can try it.

I'm a bit nervous about flexifying the parent type and then undoing that (what if it was previously flexified already?), but we can try it.

A simpler approach would be to always flexify the refined type at the top level but then to not flexify the parent type. That may unnecessarily flexify a refined type in cases when the parent is one of the types that we don't flexify (such as a primitive type), but it seems unlikely that people would want to refine such types.

I think it doesn't make sense to flexify the parent type because it doesn't make sense to refine null. You wouldn't say (Foo|null) { def bar(): Int }. Instead, you could say (Foo { def bar(): Int }) | Null.

[noti0na1]

I have run the compilation tests locally. The tests below fail because of the current awkward stage of stdlib.

tests/explicit-nulls/pos/flow-predef-eq.scala
tests/explicit-nulls/pos/eq-ne.scala
tests/explicit-nulls/warn/expose-flexible-types.scala

[WojciechMazur]

It seems to be fixing 13 projects in the OpenCB, however some of reported projects are now failing with different errors requring wrong number of arguments - I have not seen errors like that before.
All of the following projects were previously affected by FlexibleTypes match errors, there might be more but I have not yet manged to analyze their results.

• broij/zarrow - https://github.com/VirtusLab/community-build3/actions/runs/17803202877/job/50609223216#step:3:886:

Error:  166 |          check(genTuple(Gen.int, Gen.int.filter(_ != 0))) { (num, div) =>
Error:      |                                                             ^
Error:      |                               Wrong number of parameters, expected: 1
Error:  167 |            val eff = zArrow(num).provide(ZLayer.succeed(div))
Error:  168 |            assertZIO(eff)(equalTo(num / div))
Error:      |

• earogov/ordset - https://github.com/VirtusLab/community-build3/actions/runs/17803202877/job/50609223575#step:3:3881

Error:  -- [E086] Syntax Error: /build/repo/ordset/src/ordset/util/ShowUtil.scala:62:67 
Error:  62 |        val indentedLines = lines.zipWithIndex.map { (line, index) => 
Error:     |                                                     ^
Error:     |                                 Wrong number of parameters, expected: 1
Error:  63 |          if (index == 0) line else fieldIndent + line
Error:     |
Error:     | longer explanation available when compiling with `-explain`

• evolution-gaming/derivation - https://github.com/VirtusLab/community-build3/actions/runs/17803202578/job/50609227175
• facsimile/facsimile - https://github.com/VirtusLab/community-build3/actions/runs/17803202578/job/50609227214#step:3:1482

Error:  -- [E046] Cyclic Error: /build/repo/facsimile-util/src/test/scala/org/facsim/util/test/MemoizeTest.scala:224:17 
Error:  224 |          forAll { (il: List[Int]) =>
Error:      |                 ^
Error:      |                 Cyclic reference involving trait BitSet
Error:      |
Error:      |                  Run with -explain-cyclic for more details.
Error:      |

• guidoschmidt17/zio-pgcopy - https://github.com/VirtusLab/community-build3/actions/runs/17803202877/job/50609226092#step:3:963

• japgolly/univeq - https://github.com/VirtusLab/community-build3/actions/runs/17803202578/job/50609227793#step:3:869
  Previously affected by should not flixy HKTypeLambda assertion, now:

Error:  -- [E007] Type Mismatch Error: /build/repo/univeq/shared/src/main/scala-3/japgolly/univeq/internal/Derivation.scala:78:61 
Error:  78 |                log("  - Checking: " + Type.show(using fUnit.asType))
Error:     |                                                       ^^^^^^^^^^^^
Error:     |               Found:    (scala.quoted.Type[? <: AnyKind])?
Error:     |               Required: scala.quoted.Type[T]
Error:     |
Error:     |               where:    T is a type variable with constraint <: AnyKind
Error:     |
Error:     | longer explanation available when compiling with `-explain`

[olhotak]

I would guess that the errors about wrong number of arguments are due to some failure of auto-tupling probably getting confused by flexible types.