Implicit polymorphic function value not used for an implicit conversion

[noresttherein]

reproduction steps

using Scala 2.13.1,

	private[this] val cast = identity[Any] _

	abstract class TypeUnionLevel4Implicits {
		implicit def implicitUnionUnification[L, R, U <: Any | Any](implicit left :L => U, right :R => U) :(L | R) => U =
			left.asInstanceOf[(L | R) => U]
	}

	sealed abstract class TypeUnionLevel3Implicits extends TypeUnionLevel4Implicits {
		implicit def implicitRightComposedUnionMember[X, L, R <: Any | Any](implicit right :X => R) :X => (L | R) =
			right.asInstanceOf[X => (L | R)]
	}

	sealed abstract class TypeUnionLevel2Implicits extends TypeUnionLevel3Implicits {
		implicit def implicitLeftComposedUnionMember[X, L <: Any | Any, R](implicit left :X => L) :X => (L | R) =
			left.asInstanceOf[X => (L | R)]
	}

	sealed abstract class TypeUnionLevel1Implicits extends TypeUnionLevel2Implicits {
		implicit def implicitRightUnionMember[L, R] :R => (L | R) = cast.asInstanceOf[R => (L | R)]
	}

	object TypeUnion extends TypeUnionLevel1Implicits {
		implicit def implicitLeftUnionMember[L, R] :L => (L | R) = cast.asInstanceOf[L => (L | R)]

		type |[+L, +R]
	}


	implicitly[String => (String | Int)]
	implicitly[String => (Int | String)]
	val left = "left" :String | Int
	val right = "right" :Int | String
	
	implicitly[(String | Int) => (Int | String)]
	val swap = (left :String | Int) :Int | String
	implicitly[String => (Int | String | Double)]
	val middle = "middle" :Int | String | Double
	implicitly[String => (String | Int | Double | Short)]
	implicitly[String => (Short | (Double | (Int | String)))]
	val farLeft = "farLeft" :String | Int | Double | Short
	val farRight = "farRight" :Short | Double | Int | String
	implicitly[(String | Int | Double | Short) => (Short | Double | Int | String)]
	val shuffle = farLeft :Short | Double | Int | String

All lines explicitly summoning functions with implicitly compile.
None of the following lines in which these conversions should manifest (except for left and right declarations) compile.

problem

(explain how the above behavior isn't what you expected)
This goes against SLS as I understand it and certainly is very confusing.

[DmytroMitin]

https://stackoverflow.com/questions/63002466/what-are-the-hidden-rules-regarding-the-type-inference-in-resolution-of-implicit

[SethTisue]

abandoned PR in this area: scala/scala#9148