Private type member leaks in public signature through match type

## Compiler version

3.5.0-RC1

## Minimized code

```Scala
type Aux[m] = A {type M = m}
type Leak[a] = a match
  case Aux[m] => m

trait A:
  private type M
  def m: Leak[this.type]

def foo(a: A{type M = Int}): Int = a.m
```

## Output

Compiles without errors.

## Expectation

 I expected an error like a non-private method refers to a private type in its signature.