infer signatures like dotty

[adriaanm]

• prefer result type of overridden method over type of RHS of the overriding method in a subclass
• require result type for implicit definition

[som-snytt]

Completed more or less.

With and without -Xsource:3

scala> class C { def f: Option[Int] = Some(42) }
class C

scala> class D extends C { override def f = None }
class D

scala> class E extends D { override def f = Some(27) }
class E

scala>
:quit
➜  ~ scala
Welcome to Scala 2.13.10 (OpenJDK 64-Bit Server VM, Java 19).
Type in expressions for evaluation. Or try :help.

scala> class C { def f: Option[Int] = Some(42) }
class C

scala> class D extends C { override def f = None }
class D

scala> class E extends D { override def f = Some(27) }
                                                ^
       error: type mismatch;
        found   : Some[Int]
        required: None.type

Explicit types for implicits in 2.13.11 scala/scala#10083

I'd be OK if either the types were renamed or maybe just inscribed in a honorary plaque at the top of the stairs at EPFL:

class Som[A](a: A) extends Option[A]
or

type Sombyl = Symbol  // a little too close to Sybil of the 16 personalities, not Disraeli's two nations
                      // https://en.wikipedia.org/wiki/Sybil_(Schreiber_book)