Fix #7840: Don't instantiateSelected to Nothing

When instantiating type variables ahead of an implicit search, avoid instantiating
them to Nothing.

These are really two changes:

 - Assert `avoidBottom` in `instantiateSelected`
 - Change FullyDefinedAccululator logic so that `avoidBottom` overrides `minimizeAll`.

Author: odersky

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

@@ -46,7 +46,7 @@ object Inferencing {
   /** Instantiate selected type variables `tvars` in type `tp` */
   def instantiateSelected(tp: Type, tvars: List[Type])(implicit ctx: Context): Unit =
     if (tvars.nonEmpty)
-      new IsFullyDefinedAccumulator(new ForceDegree.Value(tvars.contains, minimizeAll = true)).process(tp)
+      new IsFullyDefinedAccumulator(new ForceDegree.Value(tvars.contains, minimizeAll = true, allowBottom = false)).process(tp)
 
   /** Instantiate any type variables in `tp` whose bounds contain a reference to
    *  one of the parameters in `tparams` or `vparamss`.
@@ -111,7 +111,7 @@ object Inferencing {
           def avoidBottom =
             !force.allowBottom &&
             defn.isBottomType(ctx.typeComparer.approximation(tvar.origin, fromBelow = true))
-          def preferMin = force.minimizeAll || variance >= 0 && !avoidBottom
+          def preferMin = (force.minimizeAll || variance >= 0) && !avoidBottom
           if (direction != 0) instantiate(tvar, direction < 0)
           else if (preferMin) instantiate(tvar, fromBelow = true)
           else toMaximize = true

tests/pos/i7840.scala

@@ -0,0 +1,17 @@
+object Example extends App {
+
+  trait ZSink[-R, +E, +A0, -A, +B] {
+    def flatMap[R1 <: R, E1 >: E, A00 >: A0, A1 <: A, C](
+      f: B => ZSink[R1, E1, A00, A1, C]
+    )(implicit ev: A00 =:= A1, e2: A1 =:= A00): ZSink[R1, E1, A00, A1, C] =
+      ???
+  }
+
+  object ZSink {
+    def identity[A]: ZSink[Any, Unit, Nothing, A, A] = ???
+    def succeed[A, B](b: B): ZSink[Any, Nothing, A, A, B] = ???
+  }
+
+  // cannot prove that Int =:= Nothing
+  ZSink.identity[Int].flatMap(n => ZSink.succeed[Int, String](n.toString))
+}