Upper bound info of wildcard type argument lost

[KacperFKorban]

Compiler version

3.0.0, 3.1.0, 3.1.2-RC1-bin-20220111-1292246-NIGHTLY

Minimized code

class Bound[T]
class MyClass[T <: Bound[T]]

class Container[V] {
  def doSth(): V = ???
}

def bug() = {
  val m = new Container[MyClass[_]]
  if (true) {
    m.doSth()
  }
}

Output

[error] ./main.scala:11:14: Type argument ?1.CAP does not conform to upper bound Bound[?1.CAP] in inferred type MyClass[?1.CAP]
[error]     m.doSth()
[error]

Expectation

Compile correctly.

After small changes to make the code compatible with Scala 2 (adding an outer object for def), it compiles correctly in 2.13.8.

class Bound[T]
class MyClass[T <: Bound[T]]

class Container[V] {
  def doSth(): V = ???
}

object Bug {
  def bug() = {
    val m = new Container[MyClass[_]]
    if (true) {
      m.doSth()
    }
  }
}

Workaround

The workaround for this is to add a pure expression after m.doSth(). The following code compiles:

class Bound[T]
class MyClass[T <: Bound[T]]

class Container[V] {
  def doSth(): V = ???
}

def bug() = {
  val m = new Container[MyClass[_]]
  if (true) {
    m.doSth()
    ()
  }
}

[dwijnand]

Curious what fixed this.