Fix #10980

Author: mbovel

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

@@ -1955,9 +1955,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         isSubInfo(info1, tp2.refinedInfo.widenExpr, m.symbol.info.orElse(info1))
         || matchAbstractTypeMember(m.info)
 
-      tp1.member(name) match // inlined hasAltWith for performance
+      def memberQualifies = tp1.member(name) match // inlined hasAltWith for performance
         case mbr: SingleDenotation => qualifies(mbr)
         case mbr => mbr hasAltWith qualifies
+
+      memberQualifies || (tp1.isStable && isSub(TermRef(tp1, name), tp2.refinedInfo))
     }
 
   final def ensureStableSingleton(tp: Type): SingletonType = tp.stripTypeVar match {

tests/pos/i10980.scala

@@ -0,0 +1,7 @@
+class A:
+  val x: 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