Refine minimize condition

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3930,6 +3930,10 @@ object Types {
     def hasLowerBound(implicit ctx: Context): Boolean =
       !ctx.typerState.constraint.entry(origin).loBound.isBottomType
 
+    /** For uninstantiated type variables: Is the upper bound different from Any? */
+    def hasUpperBound(implicit ctx: Context): Boolean =
+      !ctx.typerState.constraint.entry(origin).hiBound.isRef(defn.AnyClass)
+
     /** Unwrap to instance (if instantiated) or origin (if not), until result
      *  is no longer a TypeVar
      */

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

@@ -111,7 +111,9 @@ 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 && (!avoidBottom || !tvar.hasUpperBound)
+            || variance >= 0 && !avoidBottom
           if (direction != 0) instantiate(tvar, direction < 0)
           else if (preferMin) instantiate(tvar, fromBelow = true)
           else toMaximize = true

tests/neg/undet-classtag.scala

@@ -3,7 +3,7 @@ import scala.reflect.ClassTag
 object Test {
   def f[T: reflect.ClassTag](x: T) = ???
 
-  f(???)
+  f(???) // error: undetermined ClassTag
 }
 
 // SI 9754
@@ -22,6 +22,6 @@ object Program {
 
 // SI 5353
 object t5353 {
-  if (false) Array("qwe") else Array()
+  if (false) Array("qwe") else Array() // error: undetermined ClassTag
 }