Ignored upper bound of type parameter

[julienrf]

In scala/collection-strawman, this line gives the following compilation error:

def removeAll(keys: IterableOnce[K]): C = fromSpecificIterable(keys.iterator().foldLeft[CC[K, V]](coll)(_ - _))
                                                                                                        ^^^
                                 value `-` is not a member of CC[K, V]

Though CC is upper bounded by MapOps (see here), which provides the - operation. Note that the CC upper bound is recursive.

This code compiles fine with Scala 2.12 and 2.13.

[julienrf]

Here is a self-contained reduced test case: https://scastie.scala-lang.org/mlfhyDXKRDasi6imUSbFfw

Not only CC is F-bounded, but we also have the C type parameter which is upper bounded by CC.

[julienrf]

Though type C is not involved in the expression that raises the type error, if I remove it from Foo definition then the error disappears.

[odersky]

A beauty! Yet another nasty interaction between F-bounds and higher kinded types.