Remove false unchecked warnings on refined types

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

@@ -1768,7 +1768,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  Normalization is as follows: If `tp2` contains a skolem to its refinement type,
    *  rebase both itself and the member info of `tp` on a freshly created skolem type.
    */
-  protected def hasMatchingMember(name: Name, tp1: Type, tp2: RefinedType): Boolean =
+  def hasMatchingMember(name: Name, tp1: Type, tp2: RefinedType): Boolean =
     trace(i"hasMatchingMember($tp1 . $name :? ${tp2.refinedInfo}), mbr: ${tp1.member(name).info}", subtyping) {
 
       def qualifies(m: SingleDenotation): Boolean =
@@ -2750,6 +2750,9 @@ object TypeComparer {
   def matchesType(tp1: Type, tp2: Type, relaxed: Boolean)(using Context): Boolean =
     comparing(_.matchesType(tp1, tp2, relaxed))
 
+  def hasMatchingMember(name: Name, tp1: Type, tp2: RefinedType)(using Context): Boolean =
+    comparing(_.hasMatchingMember(name, tp1, tp2))
+
   def matchingMethodParams(tp1: MethodType, tp2: MethodType)(using Context): Boolean =
     comparing(_.matchingMethodParams(tp1, tp2))
 

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -150,7 +150,7 @@ object TypeTestsCasts {
       case AndType(tp1, tp2)    => recur(X, tp1) && recur(X, tp2)
       case OrType(tp1, tp2)     => recur(X, tp1) && recur(X, tp2)
       case AnnotatedType(t, _)  => recur(X, t)
-      case _: RefinedType       => false
+      case tp2: RefinedType     => recur(X, tp2.parent) && TypeComparer.hasMatchingMember(tp2.refinedName, X, tp2)
       case _                    => true
     })
 

tests/neg-custom-args/isInstanceOf/refined-types.scala

@@ -0,0 +1,22 @@
+class A
+class B extends A
+type AA = A { type T = Int }
+type BA = B { type T = Int }
+type AL = A { type T >: Int }
+type BL = B { type T >: Int }
+type AU = A { type T <: Int }
+type BU = B { type T <: Int }
+
+def aa(x: AA) = x.isInstanceOf[BA] // was: the type test for BA cannot be checked at runtime
+def al(x: AL) = x.isInstanceOf[BL] // was: the type test for BL cannot be checked at runtime
+def au(x: AU) = x.isInstanceOf[BU] // was: the type test for BU cannot be checked at runtime
+
+// an alias leaves nothing unchecked when type testing against one bound:
+def bl(x: AA) = x.isInstanceOf[BL] // was: the type test for BL cannot be checked at runtime
+def bu(x: AA) = x.isInstanceOf[BU] // was: the type test for BU cannot be checked at runtime
+
+// but static knowledge of only one bound makes checking against an alias unchecked:
+def al_ba(x: AL) = x.isInstanceOf[BA] // error: the type test for BA cannot be checked at runtime
+def au_ba(x: AU) = x.isInstanceOf[BA] // error: the type test for BA cannot be checked at runtime
+def al_bu(x: AL) = x.isInstanceOf[BU] // error: the type test for BU cannot be checked at runtime
+def au_bl(x: AU) = x.isInstanceOf[BL] // error: the type test for BL cannot be checked at runtime