avoid minimize in prefix inference

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -292,7 +292,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
   private val nullType             = ConstantType(Constant(null))
   private val nullSpace            = Typ(nullType)
 
-  override def intersectUnrelatedAtomicTypes(tp1: Type, tp2: Type) = {
+  override def intersectUnrelatedAtomicTypes(tp1: Type, tp2: Type): Space = {
     val and = AndType(tp1, tp2)
     // Precondition: !(tp1 <:< tp2) && !(tp2 <:< tp1)
     // Then, no leaf of the and-type tree `and` is a subtype of `and`.
@@ -689,7 +689,8 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
       tvar =>
         !(ctx.typerState.constraint.entry(tvar.origin) `eq` tvar.origin.underlying) ||
         (tvar `eq` removeThisType.prefixTVar),
-      minimizeAll = false
+      minimizeAll = false,
+      allowBottom = false
     )
 
     // If parent contains a reference to an abstract type, then we should

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

@@ -115,7 +115,7 @@ object Inferencing {
             val minimize =
               force.minimizeAll ||
               variance >= 0 && !(
-                force == ForceDegree.noBottom &&
+                !force.allowBottom &&
                 defn.isBottomType(ctx.typeComparer.approximation(tvar.origin, fromBelow = true)))
             if (minimize) instantiate(tvar, fromBelow = true)
             else toMaximize = true
@@ -466,9 +466,9 @@ trait Inferencing { this: Typer =>
 
 /** An enumeration controlling the degree of forcing in "is-dully-defined" checks. */
 @sharable object ForceDegree {
-  class Value(val appliesTo: TypeVar => Boolean, val minimizeAll: Boolean)
+  class Value(val appliesTo: TypeVar => Boolean, val minimizeAll: Boolean, val allowBottom: Boolean = true)
   val none = new Value(_ => false, minimizeAll = false)
   val all = new Value(_ => true, minimizeAll = false)
-  val noBottom = new Value(_ => true, minimizeAll = false)
+  val noBottom = new Value(_ => true, minimizeAll = false, allowBottom = false)
 }