Use wildcard type instead of whitebox cast in IsoFields

[mbovel]

Replace the whitebox cast (asInstanceOf[Expr[Iso[S, Tuple]]]) with a wildcard return type using PIso[S, S, ? <: Tuple, ? <: Tuple].

Since IsoFields.apply is a transparent inline method, the compiler still infers the precise tuple type at call sites. This avoids the unsafe cast.

The motivation is to prepare for future stricter checks in the Scala 3 compiler (scala/scala3#25756). The current implementation exploits a missing check to generate an unsound cast.

Note: we use PIso (a trait) directly instead of the Iso type alias (defined as type Iso[S, A] = PIso[S, S, A, A]) because Scala 3 cannot reduce higher-kinded type aliases applied to wildcard arguments. Is that acceptable to use PIso instead of Iso?

[julien-truffaut]

Hi @mbovel, sorry for the delay. I missed this PR.

Thanks for your doing it! would you mind adding the following?

1. Add a fallback match arm in IsoFieldsImpl.apply for better error messages:

  case other => report.errorAndAbort(s"Unexpected mirror type: ${other.show}")

2. adding a test for the extension syntax in IsoFieldsTest.scala

  test("fields extension method works") {
    case class Foo(s: String, i: Int)
    val foo = Foo("hi", 5)
    val iso: Iso[Foo, (String, Int)] = foo.focus.fields  // or however the syntax works
    assertEquals(iso.get(foo), ("hi", 5))
  }

[mbovel]

Thanks @julien-truffaut. I addressed your comments in 73c588b.

[julien-truffaut]

Thanks @mbovel !

[mbovel]

CI failed because of formatting; fixed, and rebased on top of main.