Singleton types are lost when used on the left of a right-associative operator

[hrhino]

From gitter a few days back:

Welcome to Scala 2.13.0-M3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_121).
Type in expressions for evaluation. Or try :help.

scala> def nom(foods: List["food"]): Unit = println("yum!")
nom: (foods: List["food"])Unit

scala> nom("food" :: Nil)
                  ^
       error: type mismatch;
        found   : rassoc$1.type (with underlying type String)
        required: "food"

scala> nom(Nil.::("food"))
yum!

The temporary ValDef that "food" is put in during desugaring gets typed with the widened String, rather than "food".

[SethTisue]

@milessabin tentative assignment, feel free to object :-)