Type inference regression in 3.1.1

[nafg]

Compiler version

3.1.1

Minimized code

  class OptGroup1[A]
  type Opt1[A] = A | OptGroup1[A]
  def Opt1[A](value: A): Opt1[A] = ???
  class WithOptions1[A]
  def creatable1[A](options: Seq[Opt1[A]]): WithOptions1[A] = new WithOptions1[A]
  val x = creatable1(Seq(Opt1("hello"), Opt1("goodbye")))
  val y: WithOptions1[String] = x

Output

[error] -- [E007] Type Mismatch Error: /home/naftoli/dev/github.com/nafg/scalajs-facades/reactSelect/src/test/scala/io/github/nafg/scalajs/facades/reactselect/CompileTimeTests.scala:18:32 
[error] 18 |  val y: WithOptions1[String] = x
[error]    |                                ^
[error]    |Found:    (io.github.nafg.scalajs.facades.reactselect.CompileTimeTests.x : 
[error]    |  io.github.nafg.scalajs.facades.reactselect.CompileTimeTests.WithOptions1[Any]
[error]    |)
[error]    |Required: io.github.nafg.scalajs.facades.reactselect.CompileTimeTests.WithOptions1[String]

Expectation

In Scala 3.1.0 it correctly infers that x is WithOptions1[String]

[smarter]

Regressed in #13650, so it looks like this only worked by chance before.

[odersky]

Here's the critical piece where it goes wrong.

We widening an inferred type as follows:

widenOr A4 | <empty>.this.OptGroup1[A4] || <empty>.this.i14420$package.Opt1[A2] = scala.this.Any in  uninstantiated variables: A4, A5, A2
 constrained types: 
  [A4]
    (options: Seq[<empty>.this.i14420$package.Opt1[A4]]): 
      <empty>.this.WithOptions1[A4]
, [A5](xs: A5*): Seq[A5], [A2](value: A2): <empty>.this.i14420$package.Opt1[A2]
 bounds: 
     A4 >: String || <empty>.this.OptGroup1[A2]
     A5
     >: A4 | <empty>.this.OptGroup1[A4^] || <empty>.this.i14420$package.Opt1[A2]
       <: 
    <empty>.this.i14420$package.Opt1[A4]
     A2 >: ("goodbye" : String) <: A4 | <empty>.this.OptGroup1[A4]

where || means soft union. We have not yet instantiated A2, so we cannot say for sure that Opt1[A2] is a subtype of OptGroup1[A4]. Hence the soft union A4 | <empty>.this.OptGroup1[A4] || <empty>.this.i14420$package.Opt1[A2] gets widened to Any.

I think the inference system works as expected here. Note that the whole problem goes away if Opt1 and OptGroup1 are made covariant.