Fall back to direct subtype comparison at the end in dropIfSuper and dropIfSub

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

@@ -2535,36 +2535,28 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   /** If some (&-operand of) `tp` is a supertype of `sub` replace it with `NoType`.
    */
   private def dropIfSuper(tp: Type, sub: Type): Type =
-
-    def isSuperOf(sub: Type): Boolean = sub match
-      case AndType(sub1, sub2) => isSuperOf(sub1) || isSuperOf(sub2)
-      case sub: TypeVar if sub.isInstantiated => isSuperOf(sub.instanceOpt)
-      case _ => isSubTypeWhenFrozen(sub, tp)
-
+    // We need to be careful to check branches of AndTypes and OrTypes in correct order,
+    // see discussion in issue #20516.
     tp match
       case tp @ AndType(tp1, tp2) =>
         recombine(dropIfSuper(tp1, sub), dropIfSuper(tp2, sub), tp)
       case tp: TypeVar if tp.isInstantiated =>
         dropIfSuper(tp.instanceOpt, sub)
       case _ =>
-        if isSuperOf(sub) then NoType else tp
+        if isSubTypeWhenFrozen(sub, tp) then NoType else tp
   end dropIfSuper
 
   /** If some (|-operand of) `tp` is a subtype of `sup` replace it with `NoType`. */
   private def dropIfSub(tp: Type, sup: Type, canConstrain: Boolean): Type =
-
-    def isSubOf(sup: Type): Boolean = sup match
-      case OrType(sup1, sup2) => isSubOf(sup1) || isSubOf(sup2)
-      case sup: TypeVar if sup.isInstantiated => isSubOf(sup.instanceOpt)
-      case _ => isSubType(tp, sup, whenFrozen = !canConstrain)
-
+    // We need to be careful to check branches of AndTypes and OrTypes in correct order,
+    // see discussion in issue #20516.
     tp match
       case tp @ OrType(tp1, tp2) =>
         recombine(dropIfSub(tp1, sup, canConstrain), dropIfSub(tp2, sup, canConstrain), tp)
       case tp: TypeVar if tp.isInstantiated =>
         dropIfSub(tp.instanceOpt, sup, canConstrain)
       case _ =>
-        if isSubOf(sup) then NoType else tp
+        if isSubType(tp, sup, whenFrozen = !canConstrain) then NoType else tp
   end dropIfSub
 
   /** There's a window of vulnerability between ElimByName and Erasure where some

tests/pos-deep-subtype/i20516.scala

@@ -0,0 +1,205 @@
+object Main {
+  trait A {}
+  trait B {}
+  trait C {}
+  trait D {}
+  trait E {}
+  trait F {}
+  trait G {}
+  trait H {}
+  trait I {}
+  trait J {}
+  trait K {}
+  trait L {}
+  trait M {}
+  trait N {}
+  trait O {}
+  trait P {}
+  trait Q {}
+  trait R {}
+  trait S {}
+  trait T {}
+  trait U {}
+  trait V {}
+  trait W {}
+  trait X {}
+  trait Y {}
+  trait Z {}
+
+  type AlphabeticServices = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+
+  type EnvOutA = B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutB = A & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutC = A & B & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutD = A & B & C & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutE = A & B & C & D & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutF = A & B & C & D & E & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutG = A & B & C & D & E & F & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutH = A & B & C & D & E & F & G & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutI = A & B & C & D & E & F & G & H & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutJ = A & B & C & D & E & F & G & H & I & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutK = A & B & C & D & E & F & G & H & I & J & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutL = A & B & C & D & E & F & G & H & I & J & K & M & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutM = A & B & C & D & E & F & G & H & I & J & K & L & N & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutN = A & B & C & D & E & F & G & H & I & J & K & L & M & O & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutO = A & B & C & D & E & F & G & H & I & J & K & L & M & N & P & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutP = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & Q & R & S & T & U & V & W & X & Y & Z
+  type EnvOutQ = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & R & S & T & U & V & W & X & Y & Z
+  type EnvOutR = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & S & T & U & V & W & X & Y & Z
+  type EnvOutS = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & T & U & V & W & X & Y & Z
+  type EnvOutT = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & U & V & W & X & Y & Z
+  type EnvOutU = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & V & W & X & Y & Z
+  type EnvOutV = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & W & X & Y & Z
+  type EnvOutW = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & X & Y & Z
+  type EnvOutX = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & Y & Z
+  type EnvOutY = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Z
+  type EnvOutZ = A & B & C & D & E & F & G & H & I & J & K & L & M & N & O & P & Q & R & S & T & U & V & W & X & Y
+
+  trait Reader[-E, A] {
+    def map[B](f: A => B): Reader[E, B] = ???
+    def flatMap[E2 <: E, B](f: A => Reader[E2, B]): Reader[E2, B] = ???
+  }
+
+  def e1: Reader[EnvOutA, Unit] = ???
+  def e2: Reader[EnvOutB, Unit] = ???
+  def e3: Reader[EnvOutC, Unit] = ???
+  def e4: Reader[EnvOutD, Unit] = ???
+  def e5: Reader[EnvOutE, Unit] = ???
+  def e6: Reader[EnvOutF, Unit] = ???
+  def e7: Reader[EnvOutG, Unit] = ???
+  def e8: Reader[EnvOutH, Unit] = ???
+  def e9: Reader[EnvOutI, Unit] = ???
+  def e10: Reader[EnvOutJ, Unit] = ???
+  def e11: Reader[EnvOutK, Unit] = ???
+  def e12: Reader[EnvOutL, Unit] = ???
+  def e13: Reader[EnvOutM, Unit] = ???
+  def e14: Reader[EnvOutN, Unit] = ???
+  def e15: Reader[EnvOutO, Unit] = ???
+  def e16: Reader[EnvOutP, Unit] = ???
+  def e17: Reader[EnvOutQ, Unit] = ???
+  def e18: Reader[EnvOutR, Unit] = ???
+  def e19: Reader[EnvOutS, Unit] = ???
+  def e20: Reader[EnvOutT, Unit] = ???
+  def e21: Reader[EnvOutU, Unit] = ???
+  def e22: Reader[EnvOutV, Unit] = ???
+  def e23: Reader[EnvOutW, Unit] = ???
+  def e24: Reader[EnvOutX, Unit] = ???
+  def e25: Reader[EnvOutY, Unit] = ???
+  def e26: Reader[EnvOutZ, Unit] = ???
+
+  def program: Reader[AlphabeticServices, Unit] = for {
+    //1
+    _ <- e1
+    _ <- e2
+    _ <- e3
+    _ <- e4
+    _ <- e5
+    _ <- e6
+    _ <- e7
+    _ <- e8
+    _ <- e8
+    _ <- e9
+    _ <- e10
+    _ <- e11
+    _ <- e12
+    _ <- e13
+    _ <- e14
+    _ <- e15
+    _ <- e16
+    _ <- e17
+    _ <- e18
+    _ <- e19
+    _ <- e20
+    _ <- e21
+    _ <- e22
+    _ <- e23
+    _ <- e24
+    _ <- e25
+    _ <- e26
+    // 2
+    _ <- e1
+    _ <- e2
+    _ <- e3
+    _ <- e4
+    _ <- e5
+    _ <- e6
+    _ <- e7
+    _ <- e8
+    _ <- e8
+    _ <- e9
+    _ <- e10
+    _ <- e11
+    _ <- e12
+    _ <- e13
+    _ <- e14
+    _ <- e15
+    _ <- e16
+    _ <- e17
+    _ <- e18
+    _ <- e19
+    _ <- e20
+    _ <- e21
+    _ <- e22
+    _ <- e23
+    _ <- e24
+    _ <- e25
+    _ <- e26
+    // TODO: optimize the subtype checking for large intersection types further
+    //3
+    // _ <- e1
+    // _ <- e2
+    // _ <- e3
+    // _ <- e4
+    // _ <- e5
+    // _ <- e6
+    // _ <- e7
+    // _ <- e8
+    // _ <- e8
+    // _ <- e9
+    // _ <- e10
+    // _ <- e11
+    // _ <- e12
+    // _ <- e13
+    // _ <- e14
+    // _ <- e15
+    // _ <- e16
+    // _ <- e17
+    // _ <- e18
+    // _ <- e19
+    // _ <- e20
+    // _ <- e21
+    // _ <- e22
+    // _ <- e23
+    // _ <- e24
+    // _ <- e25
+    // _ <- e26
+    // 4
+    // _ <- e1
+    // _ <- e2
+    // _ <- e3
+    // _ <- e4
+    // _ <- e5
+    // _ <- e6
+    // _ <- e7
+    // _ <- e8
+    // _ <- e8
+    // _ <- e9
+    // _ <- e10
+    // _ <- e11
+    // _ <- e12
+    // _ <- e13
+    // _ <- e14
+    // _ <- e15
+    // _ <- e16
+    // _ <- e17
+    // _ <- e18
+    // _ <- e19
+    // _ <- e20
+    // _ <- e21
+    // _ <- e22
+    // _ <- e23
+    // _ <- e24
+    // _ <- e25
+    // _ <- e26
+    } yield ()
+}