Add explicit type annotations to implicits

Always add an explicit type to your implicit values or methods, even when they’re private

Reason

Aside from binary compatibility concerns, implicit resolution is complex and, sometimes, buggy.

The following code compiles:

trait Foo[_]
class Bar

implicitly[Foo[Bar]]

object Bar {
  implicit val impFoo: Foo[Bar] = new Foo[Bar] {}
}

impFoo is in the implicit scope of Foo[Bar], and our implicitly[Foo[Bar]] resolves.

But, because of bug 8697, the following does not:

trait Foo[_]
class Bar

implicitly[Foo[Bar]]

object Bar {
  implicit val impFoo = new Foo[Bar] {}
}
// error: could not find implicit value for parameter e: repl.Session.App0.Foo[repl.Session.App0.Bar]
// implicitly[Foo[Bar]]
// ^^^^^^^^^^^^^^^^^^^^

The only difference between the two is that impFoo has an explicit type in the first snippet but not in the second.

The behaviour is clearly counter-intuitive, and considered to be a bug - but it results in a compilation failure that’s trivial to fix once you know the problem.

Well. Most of the time. There are cases where it will result in the wrong implicit value being picked, which is a lot harder to realise and fix.

Checked by