-Ycheck reports false positives for existentials in the expected type

[sjrd]

Main.scala:

package test

object Main {
  def main(args: Array[String]): Unit = {
    val c: Class[_] = classOf[String]
    println(c)
    foo(classOf[String])
  }

  def foo(c: Class[_]): Unit =
    println(c)
}

Invocation:

$ ~/opt/scala-2.12.3/bin/scalac -Ycheck:typer,tailcalls Main.scala 
[Now checking: typer]
warning: [check: typer]
Out of scope symbol reference {
      tree  ValDef               val c: Class[_] = classOf[java.lang.String]
  position  RangePosition        Main.scala:5
  with sym  TermSymbol           val c: Class[_]                              in  method main -> object Main -> package test
   encl(1)  MethodSymbol         def main(args: Array[String]): Unit          in  object Main -> package test
   encl(2)  ModuleSymbol         object Main                                  in  package test
    ref to  AbstractTypeSymbol   _$1 (<deferred> <existential>)               in  value c -> method main -> object Main -> package test
}
warning: [check: typer]
Out of scope symbol reference {
      tree  Ident                c
  position  OffsetPosition       Main.scala:6
  with sym  TermSymbol           val c: Class[_]                      in  method main -> object Main -> package test
   and tpe  AliasArgsTypeRef     Class[_$1]
   encl(1)  MethodSymbol         def main(args: Array[String]): Unit  in  object Main -> package test
   encl(2)  ModuleSymbol         object Main                          in  package test
    ref to  AbstractTypeSymbol   _$1 (<deferred> <existential>)       in  value c -> method main -> object Main -> package test
}
warning: [check: typer]
Out of scope symbol reference {
      tree  Apply                Main.this.foo(classOf[java.lang.String])
  position  OffsetPosition       Main.scala:7
  with sym  MethodSymbol         def foo(c: Class[_]): Unit                in  object Main -> package test
   and tpe  ClassNoArgsTypeRef   Unit
   encl(1)  MethodSymbol         def main(args: Array[String]): Unit       in  object Main -> package test
   encl(2)  ModuleSymbol         object Main                               in  package test
    ref to  AbstractTypeSymbol   _$2 (<deferred> <existential>)            in  value c -> method foo -> object Main -> package test
}
warning: [check: typer]
Out of scope symbol reference {
      tree  Select               Main.this.foo
  position  OffsetPosition       Main.scala:7
  with sym  MethodSymbol         def foo(c: Class[_]): Unit           in  object Main -> package test
   and tpe  MethodType           (c: Class[_])Unit
   encl(1)  MethodSymbol         def main(args: Array[String]): Unit  in  object Main -> package test
   encl(2)  ModuleSymbol         object Main                          in  package test
    ref to  AbstractTypeSymbol   _$2 (<deferred> <existential>)       in  value c -> method foo -> object Main -> package test
}
warning: [check: typer]
Out of scope symbol reference {
      tree  DefDef               def foo(c: Class[_]): Unit = scala.Predef.println(c)
  position  OffsetPosition       Main.scala:10
  with sym  MethodSymbol         def foo(c: Class[_]): Unit                            in  object Main -> package test
   encl(1)  ModuleSymbol         object Main                                           in  package test
    ref to  AbstractTypeSymbol   _$2 (<deferred> <existential>)                        in  value c -> method foo -> object Main -> package test
}
warning: [check: typer]
Out of scope symbol reference {
      tree  ValDef               val c: Class[_] = _
  position  OffsetPosition       Main.scala:10
  with sym  TermSymbol           c: Class[_]                     in  method foo -> object Main -> package test
   encl(1)  MethodSymbol         def foo(c: Class[_]): Unit      in  object Main -> package test
   encl(2)  ModuleSymbol         object Main                     in  package test
    ref to  AbstractTypeSymbol   _$2 (<deferred> <existential>)  in  value c -> method foo -> object Main -> package test
}
warning: [check: typer]
Out of scope symbol reference {
      tree  Ident                c
  position  OffsetPosition       Main.scala:11
  with sym  TermSymbol           c: Class[_]                     in  method foo -> object Main -> package test
   and tpe  AliasArgsTypeRef     Class[_$2]
   encl(1)  MethodSymbol         def foo(c: Class[_]): Unit      in  object Main -> package test
   encl(2)  ModuleSymbol         object Main                     in  package test
    ref to  AbstractTypeSymbol   _$2 (<deferred> <existential>)  in  value c -> method foo -> object Main -> package test
}
[Now checking: tailcalls]
warning: [check: tailcalls]
Out of scope symbol reference {
      tree  Apply                Main.this.foo(classOf[java.lang.String])
  position  OffsetPosition       Main.scala:7
  with sym  MethodSymbol         def foo(c: Class[_]): Unit                in  object Main -> package test
   and tpe  ClassNoArgsTypeRef   Unit
   encl(1)  MethodSymbol         def main(args: Array[String]): Unit       in  object Main -> package test
   encl(2)  ModuleClassSymbol    Main: test.Main.type                      in  package test
    ref to  AbstractTypeSymbol   _$2 (<deferred> <existential>)            in  value c -> method foo -> object Main -> package test
}
warning: [check: tailcalls]
Out of scope symbol reference {
      tree  Select               Main.this.foo
  position  OffsetPosition       Main.scala:7
  with sym  MethodSymbol         def foo(c: Class[_]): Unit           in  object Main -> package test
   and tpe  MethodType           (c: Class[_])Unit
   encl(1)  MethodSymbol         def main(args: Array[String]): Unit  in  object Main -> package test
   encl(2)  ModuleClassSymbol    Main: test.Main.type                 in  package test
    ref to  AbstractTypeSymbol   _$2 (<deferred> <existential>)       in  value c -> method foo -> object Main -> package test
}
warning: [check: tailcalls]
Out of scope symbol reference {
      tree  DefDef               def foo(c: Class[_]): Unit = scala.Predef.println(c)
  position  OffsetPosition       Main.scala:10
  with sym  MethodSymbol         def foo(c: Class[_]): Unit                            in  object Main -> package test
   encl(1)  ModuleClassSymbol    Main: test.Main.type                                  in  package test
    ref to  AbstractTypeSymbol   _$2 (<deferred> <existential>)                        in  value c -> method foo -> object Main -> package test
}
10 warnings found

The same issue can be reproduced, among others, with 2.12.4, 2.11.8, 2.11.11, and 2.10.6 (the latter with less verbose messages, but same error it seems).