Always minimize type variables when interpolating an expected type of an implicit.

This was suggested in #878.

Author: odersky

src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -1611,7 +1611,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
           // (otherwise equality is required)
           def compareOp: (Tree, Tree) => Tree =
             if (aligner.isStar) _.select(defn.Int_>=).appliedTo(_)
-            else         _.select(defn.Int_==).appliedTo(_)
+            else _.select(defn.Int_==).appliedTo(_)
 
           // `if (binder != null && $checkExpectedLength [== | >=] 0) then else zero`
           (seqTree(binder).select(defn.Any_!=).appliedTo(Literal(Constant(null)))).select(defn.Boolean_&&).appliedTo(compareOp(checkExpectedLength, Literal(Constant(0))))

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 =
-    new IsFullyDefinedAccumulator(new ForceDegree.Value(tvars.contains)).process(tp)
+    new IsFullyDefinedAccumulator(new ForceDegree.Value(tvars.contains, minimizeAll = true)).process(tp)
 
   /** The accumulator which forces type variables using the policy encoded in `force`
    *  and returns whether the type is fully defined. The direction in which
@@ -81,11 +81,12 @@ object Inferencing {
         force.appliesTo(tvar) && {
           val direction = instDirection(tvar.origin)
           if (direction != 0) {
-            //if (direction > 0) println(s"inst $tvar dir = up")
+            if (direction > 0) println(s"inst $tvar dir = up")
             instantiate(tvar, direction < 0)
           }
           else {
             val minimize =
+              force.minimizeAll ||
               variance >= 0 && !(
                 force == ForceDegree.noBottom &&
                 defn.isBottomType(ctx.typeComparer.approximation(tvar.origin, fromBelow = true)))
@@ -293,9 +294,9 @@ object Inferencing {
 
 /** An enumeration controlling the degree of forcing in "is-dully-defined" checks. */
 @sharable object ForceDegree {
-  class Value(val appliesTo: TypeVar => Boolean)
-  val none = new Value(_ => false)
-  val all = new Value(_ => true)
-  val noBottom = new Value(_ => true)
+  class Value(val appliesTo: TypeVar => Boolean, val minimizeAll: Boolean)
+  val none = new Value(_ => false, minimizeAll = false)
+  val all = new Value(_ => true, minimizeAll = false)
+  val noBottom = new Value(_ => true, minimizeAll = false)
 }