can't define right associative extension method with type parameter

## Compiler version

3.0.0-RC1

## Minimized code

```Scala
extension (s: String)
  def :*:[T <: Tuple](that: T) : String *: T = ???
```

## Output

```scala
2 |  def :*:[T <: Tuple](that: T) : String *: T = ???
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |  right-associative extension method must start with a single parameter
```

## Expectation

no error