Go to definition doesn't work for singleton operations

[jiribenes]

effect raise(): Unit
def example(): Unit / raise = do raise()

Go to definition doesn't work:

• neither for the effect annotation (the / raise part),
• nor for the effect operation (the do raise()).

Both return an unfriendly pop-up:

---

However it works perfectly fine for interfaces and their operations:

interface Exc { def raise(): Unit }
       // ~~~

def example(): Unit / Exc = do raise()

Here, go to definition on both / Exc and do raise() jumps to the ~~~ highlighted area above.

Relevant code

I think there's some missing position info here:

effekt/effekt/shared/src/main/scala/effekt/RecursiveDescent.scala

Lines 469 to 486 in 6e71bcd

	// effect <NAME>[...](...): ...
	def operationDef(): Def =
	nonterminal:
	`effect` ~> operation() match {
	case op @ Operation(id, tps, vps, bps, ret) =>
	// TODO is the `true` flag used at all anymore???
	InterfaceDef(IdDef(id.name), tps, List(Operation(id, Nil, vps, bps, ret) withPositionOf op))
	}
	def operation(): Operation =
	nonterminal:
	idDef() ~ params() ~ returnAnnotation() match {
	case id ~ (tps, vps, bps) ~ ret => Operation(id, tps, vps, bps, ret)
	}
	def interfaceDef(): InterfaceDef =
	nonterminal:
	InterfaceDef(`interface` ~> idDef(), maybeTypeParams(), `{` ~> manyWhile(`def` ~> operation(), `def`) <~ `}`)