type arguments do not conform to trait Subtractable's type parameter bounds

[Proximator]

After upgrading to Scala 2.12, I am having a weird compilation issue of this code:

val c = Map("a" -> Set[Map[String, String]](), "b" -> Map[String, String]())

It is throwing this exception:
Error:(19, 7) type arguments [?,Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any with scala.collection.generic.Subtractable[_ >: String with scala.collection.immutable.Map[String,String] <: Object, Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any with scala.collection.generic.Subtractable[_ >: String with scala.collection.immutable.Map[String,String] <: Object, Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any with scala.collection.generic.Subtractable[_ >: String with scala.collection.immutable.Map[String,String] <: Object, Equals]]{def seq: Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any with scala.collection.generic.Subtractable[_ >: String with scala.collection.immutable.Map[String,String] <: Object, Equals]}]{def seq: Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any with scala.collection.generic.Subtractable[_ >: String with scala.collection.immutable.Map[String,String] <: Object, Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any with scala.collection.generic.Subtractable[_ >: String with scala.collection.immutable.Map[String,String] <: Object, Equals]]{def seq: Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any with scala.collection.generic.Subtractable[_ >: String with scala.collection.immutable.Map[String,String] <: Object, Equals]}}] do not conform to trait Subtractable's type parameter bounds [A,+Repr <: scala.collection.generic.Subtractable[A,Repr]]

[som-snytt]

Probably #10151

[SethTisue]

note that even in 2.11 a rather outlandish type is inferred:

Welcome to Scala 2.11.12 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_162).

scala> val c = Map("a" -> Set[Map[String, String]](), "b" -> Map[String, String]())
c: scala.collection.immutable.Map[String,scala.collection.immutable.Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any{def seq: scala.collection.immutable.Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any{def seq: scala.collection.immutable.Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any{def seq: scala.collection.immutable.Iterable[Equals] with String with scala.collection.immutable.Map[String,String] => Any}}}] = Map(a -> Set(), b -> Map())

let's consolidate under #10151