Add isBottomType/Class tests to Definitions

Author: odersky

src/dotty/tools/dotc/core/Definitions.scala

@@ -588,6 +588,12 @@ class Definitions {
     name.startsWith(prefix) && name.drop(prefix.length).forall(_.isDigit)
   }
 
+  def isBottomClass(cls: Symbol) = cls == NothingClass || cls == NullClass
+  def isBottomType(tp: Type) = tp match {
+    case tp: TypeRef => isBottomClass(tp.symbol)
+    case _ => false
+  }
+
   def isFunctionClass(cls: Symbol) = isVarArityClass(cls, tpnme.Function)
   def isAbstractFunctionClass(cls: Symbol) = isVarArityClass(cls, tpnme.AbstractFunction)
   def isTupleClass(cls: Symbol) = isVarArityClass(cls, tpnme.Tuple)

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

@@ -89,7 +89,7 @@ object Inferencing {
             val minimize =
               variance >= 0 && !(
                 force == ForceDegree.noBottom &&
-                isBottomType(ctx.typeComparer.approximation(tvar.origin, fromBelow = true)))
+                defn.isBottomType(ctx.typeComparer.approximation(tvar.origin, fromBelow = true)))
             if (minimize) instantiate(tvar, fromBelow = true)
             else toMaximize = true
           }
@@ -173,9 +173,6 @@ object Inferencing {
     approxAbove - approxBelow
   }
 
-  def isBottomType(tp: Type)(implicit ctx: Context) =
-    tp == defn.NothingType || tp == defn.NullType
-
   /** Recursively widen and also follow type declarations and type aliases. */
   def widenForMatchSelector(tp: Type)(implicit ctx: Context): Type = tp.widen match {
     case tp: TypeRef if !tp.symbol.isClass => widenForMatchSelector(tp.info.bounds.hi)