No murmur from pattern match of singleton type of Any

[som-snytt]

Compiler version

3.1.3-RC1

Minimized code

class C[A <: AnyRef] { val a: A = null.asInstanceOf[A] ; def f(x: A) = x match { case _: a.type => 42 } }
class D[A] { val a: A = null.asInstanceOf[A] ; def f(x: A) = x match { case _: a.type => 42 } }

Output

scala> class C[A] { val a: A = null.asInstanceOf[A] ; def f(x: A) = x match { case _: a.type => 42 } }
// defined class C

scala> class C[A <: AnyRef] { val a: A = null.asInstanceOf[A] ; def f(x: A) = x match { case _: a.type => 42 } }
// defined class C

Expectation

scala 2.13.8> class C[A] { val a: A = null.asInstanceOf[A] ; def f(x: A) = x match { case _: a.type => 42 } }
                                                                                              ^
              warning: abstract type pattern A (the underlying of C.this.a.type) is unchecked since it is eliminated by erasure
class C

Some sort of messaging that my singleton type pattern is not kosher and will not be checked with eq.

With the upper bound, eq is used as expected. That is the same behavior as Scala 2, which murmurs about it.

Forward port of scala/bug#12569

[som-snytt]

Duplicates #9359 and related