Fix #3782: Fix type parameter inference with scala.Singleton

The fix in ec53918 was incomplete: we
need to check if the upper bound is a subtype and not just a reference
to scala.Singleton

Author: smarter

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

@@ -272,10 +272,10 @@ trait ConstraintHandling {
 
     // Then, approximate by (1.) - (3.) and simplify as follows.
     // 1. If instance is from below and is a singleton type, yet upper bound is
-    // not a singleton type or a reference to `scala.Singleton`, widen the
+    // not a singleton type or a subtype of `scala.Singleton`, widen the
     // instance.
     if (fromBelow && isMultiSingleton(inst) && !isMultiSingleton(upperBound)
-        && !upperBound.isRef(defn.SingletonClass))
+        && !isSubTypeWhenFrozen(upperBound, defn.SingletonType))
       inst = inst.widen
 
     // 2. If instance is from below and is a fully-defined union type, yet upper bound

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

@@ -384,6 +384,7 @@ class Definitions {
     enterCompleteClassSymbol(
       ScalaPackageClass, tpnme.Singleton, PureInterfaceCreationFlags | Final,
       List(AnyClass.typeRef), EmptyScope)
+  lazy val SingletonType: TypeRef = SingletonClass.typeRef
 
   lazy val SeqType: TypeRef = ctx.requiredClassRef("scala.collection.Seq")
   def SeqClass(implicit ctx: Context) = SeqType.symbol.asClass

tests/pos/singletontrait.scala

@@ -1,5 +1,7 @@
 object Test {
   def foo[T <: Singleton](x: T): T = x
+  def bar[T <: Int with Singleton](x: T): T = x
 
   val a: 1 = foo(1)
+  val b: 1 = bar(1)
 }