Avoid double negation in isJvmSam.

Found while trying to chase down the problem with stale symbols in last commit. Double negation
is confusing. The new formulation avoids it.

Author: odersky

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

@@ -25,20 +25,19 @@ class ExpandSAMs extends MiniPhaseTransform { thisTransformer =>
 
   import ast.tpd._
 
-  def noJvmSam(cls: ClassSymbol)(implicit ctx: Context): Boolean =
-    !cls.is(Trait) ||
-    cls.superClass != defn.ObjectClass ||
-    !cls.is(NoInits) ||
-    !cls.directlyInheritedTraits.forall(_.is(NoInits)) ||
-    ExplicitOuter.needsOuterIfReferenced(cls) ||
-    cls.typeRef.fields.nonEmpty // Superaccessors already show up as abstract methods here, so no test necessary
-
+  /** Is SAMType `cls` also a SAM under the rules of the JVM? */
+  def isJvmSam(cls: ClassSymbol)(implicit ctx: Context): Boolean =
+    cls.is(NoInitsTrait) &&
+    cls.superClass == defn.ObjectClass &&
+    cls.directlyInheritedTraits.forall(_.is(NoInits)) &&
+    !ExplicitOuter.needsOuterIfReferenced(cls) &&
+    cls.typeRef.fields.isEmpty // Superaccessors already show up as abstract methods here, so no test necessary
 
   override def transformBlock(tree: Block)(implicit ctx: Context, info: TransformerInfo): Tree = tree match {
     case Block(stats @ (fn: DefDef) :: Nil, Closure(_, fnRef, tpt)) if fnRef.symbol == fn.symbol =>
       tpt.tpe match {
         case NoType => tree // it's a plain function
-        case tpe @ SAMType(_) if !noJvmSam(tpe.classSymbol.asClass) =>
+        case tpe @ SAMType(_) if isJvmSam(tpe.classSymbol.asClass) =>
           if (tpe isRef defn.PartialFunctionClass) toPartialFunction(tree)
           else tree
         case tpe =>