Make singleton types subtypes of their refined type (#16135)

Author: mbovel

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

@@ -1954,6 +1954,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         val info1 = m.info.widenExpr
         isSubInfo(info1, tp2.refinedInfo.widenExpr, m.symbol.info.orElse(info1))
         || matchAbstractTypeMember(m.info)
+        || (tp1.isStable && isSubType(TermRef(tp1, m.symbol), tp2.refinedInfo))
 
       tp1.member(name) match // inlined hasAltWith for performance
         case mbr: SingleDenotation => qualifies(mbr)

tests/pos/i10980.scala

@@ -0,0 +1,9 @@
+class A:
+  val x: AnyRef = Object()
+  val y: AnyRef = Object()
+
+@main def m: Unit =
+  val a = new A
+  val b: a.x.type = a.x
+  val c: A { val x: a.x.type } = a
+  val d: A { val x: a.x.type;  val y: a.y.type } = a