Undo #7130

Undo the change where we do not interpolate a type variable if
its lower bound is nothing. It turned out this broke type inference.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -433,14 +433,7 @@ trait Inferencing { this: Typer =>
             else if (!hasUnreportedErrors)
               if (v.intValue != 0) {
                 typr.println(i"interpolate $tvar in $state in $tree: $tp, fromBelow = ${v.intValue == 1}, $constraint")
-                if (true) {
-                  val fromBelow = v.intValue == 1
-                  val instType = ctx.typeComparer.instanceType(tvar.origin, fromBelow)
-                  if (!(fromBelow && instType.isRef(defn.NothingClass)))
-                    tvar.instantiateWith(instType)
-                }
-                else
-                  tvar.instantiate(fromBelow = v.intValue == 1)
+                tvar.instantiate(fromBelow = v.intValue == 1)
               }
               else typr.println(i"no interpolation for nonvariant $tvar in $state")
           }

tests/pending/pos/enum-interop.scala renamed to tests/pos/enum-interop.scala

@@ -0,0 +1,20 @@
+trait ZIO[-R, +E, +A] { self =>
+
+  final def +++[R1, B, E1 >: E](that: ZIO[R1, E1, B]): ZIO[Either[R, R1], E1, Either[A, B]] =
+    for {
+      e <- ZIO.environment[Either[R, R1]]
+      r <- e.fold(self.map(Left(_)) provide _, that.map(Right(_)) provide _)
+    } yield r
+    // Found:    (Left[A, Any] | Right[Any, B])(r)
+    // Required: Either[A, B]
+
+  def flatMap[R1 <: R, E1 >: E, B](f: A => ZIO[R1, E1, B]): ZIO[R1, E1, B] = ???
+
+  def map[B](f: A => B): ZIO[R, E, B] = ???
+
+  def provide[R](R: R): ZIO[Any, E, A] = ???
+}
+
+object ZIO {
+  def environment[R]: ZIO[R, Nothing, R] = ???
+}