Optimize isSameType for invariant applied types with the same structure

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

@@ -8,8 +8,7 @@ import Phases.{gettersPhase, elimByNamePhase}
 import StdNames.nme
 import TypeOps.refineUsingParent
 import collection.mutable
-import util.Stats
-import util.NoSourcePosition
+import util.{Stats, NoSourcePosition, EqHashMap}
 import config.Config
 import config.Feature.migrateTo3
 import config.Printers.{subtyping, gadts, matchTypes, noPrinter}
@@ -163,6 +162,20 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   /** A flag to prevent recursive joins when comparing AndTypes on the left */
   private var joined = false
 
+  /** A variable to keep track of number of outstanding isSameType tests */
+  private var sameLevel = 0
+
+  /** A map that records successful isSameType comparisons.
+   *  Used together with `sameLevel` to avoid exponential blowUp of isSameType
+   *  comparisons for deeply nested invariant applied types.
+   */
+  private var sames: util.EqHashMap[Type, Type] | Null = null
+
+  /** The `sameLevel` nesting depth from which on we want to keep track
+   *  of isSameTypes suucesses using `sames`
+   */
+  val startSameTypeTrackingLevel = 3
+
   private inline def inFrozenGadtIf[T](cond: Boolean)(inline op: T): T = {
     val savedFrozenGadt = frozenGadt
     frozenGadt ||= cond
@@ -1553,8 +1566,9 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                    && defn.isByNameFunction(arg2.dealias) =>
                  isSubArg(arg1res, arg2.argInfos.head)
               case _ =>
-                (v > 0 || isSubType(arg2, arg1)) &&
-                (v < 0 || isSubType(arg1, arg2))
+                if v < 0 then isSubType(arg2, arg1)
+                else if v > 0 then isSubType(arg1, arg2)
+                else isSameType(arg2, arg1)
 
         isSubArg(args1.head, args2.head)
       } && recurArgs(args1.tail, args2.tail, tparams2.tail)
@@ -2012,11 +2026,28 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
   // Type equality =:=
 
-  /** Two types are the same if are mutual subtypes of each other */
+  /** Two types are the same if they are mutual subtypes of each other.
+   *  To avoid exponential blowup for deeply nested invariant applied types,
+   *  we cache successes once the stack of outstanding isSameTypes reaches
+   *  depth `startSameTypeTrackingLevel`. See pos/i15525.scala, where this matters.
+   */
   def isSameType(tp1: Type, tp2: Type): Boolean =
-    if (tp1 eq NoType) false
-    else if (tp1 eq tp2) true
-    else isSubType(tp1, tp2) && isSubType(tp2, tp1)
+    if tp1 eq NoType then false
+    else if tp1 eq tp2 then true
+    else if sames != null && (sames.nn.lookup(tp1) eq tp2) then true
+    else
+      val savedSames = sames
+      sameLevel += 1
+      if sameLevel >= startSameTypeTrackingLevel then
+        Stats.record("cache same type")
+        sames = new util.EqHashMap()
+      val res =
+        try isSubType(tp1, tp2) && isSubType(tp2, tp1)
+        finally
+          sameLevel -= 1
+          sames = savedSames
+      if res && sames != null then sames.nn(tp2) = tp1
+      res
 
   override protected def isSame(tp1: Type, tp2: Type)(using Context): Boolean = isSameType(tp1, tp2)
 

tests/neg/i15525.scala

@@ -0,0 +1,53 @@
+class /[D, T]
+class Delegating[D]
+
+type Aux[E] = Container { type Elements = E }
+
+class Container:
+  type Elements = Delegating[Delegates]
+  type Delegates
+
+class Resolution[E](value: Aux[E]):
+  type Type = Aux[E]
+
+def element22(
+    transmittable0: Resolution[?], transmittable1: Resolution[?],
+    transmittable2: Resolution[?], transmittable3: Resolution[?],
+    transmittable4: Resolution[?], transmittable5: Resolution[?],
+    transmittable6: Resolution[?], transmittable7: Resolution[?],
+    transmittable8: Resolution[?], transmittable9: Resolution[?],
+    transmittable10: Resolution[?], transmittable11: Resolution[?],
+    transmittable12: Resolution[?], transmittable13: Resolution[?],
+    transmittable14: Resolution[?], transmittable15: Resolution[?],
+    transmittable16: Resolution[?], transmittable17: Resolution[?],
+    transmittable18: Resolution[?], transmittable19: Resolution[?],
+    transmittable20: Resolution[?], transmittable21: Resolution[?])
+: Container {
+    type Delegates =
+      transmittable0.Type / transmittable1.Type /
+      transmittable2.Type / transmittable3.Type /
+      transmittable4.Type / transmittable5.Type /
+      transmittable6.Type / transmittable7.Type /
+      transmittable8.Type / transmittable9.Type /
+      transmittable10.Type / transmittable11.Type /
+      transmittable12.Type / transmittable13.Type /
+      transmittable14.Type / transmittable15.Type /
+      transmittable16.Type / transmittable17.Type /
+      transmittable18.Type / transmittable19.Type /
+      transmittable20.Type / transmittable21.Type
+  } = ???
+
+def test22 =
+  Resolution(
+    element22(
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0), // error // error
+      Resolution(element0), Resolution(element0)))// error // error

tests/pos/i15525.scala

@@ -0,0 +1,55 @@
+class /[D, T]
+class Delegating[D]
+
+type Aux[E] = Container { type Elements = E }
+
+class Container:
+  type Elements = Delegating[Delegates]
+  type Delegates
+
+class Resolution[E](value: Aux[E]):
+  type Type = Aux[E]
+
+def element0: Container { type Delegates = Unit } = ???
+
+def element22(
+    transmittable0: Resolution[?], transmittable1: Resolution[?],
+    transmittable2: Resolution[?], transmittable3: Resolution[?],
+    transmittable4: Resolution[?], transmittable5: Resolution[?],
+    transmittable6: Resolution[?], transmittable7: Resolution[?],
+    transmittable8: Resolution[?], transmittable9: Resolution[?],
+    transmittable10: Resolution[?], transmittable11: Resolution[?],
+    transmittable12: Resolution[?], transmittable13: Resolution[?],
+    transmittable14: Resolution[?], transmittable15: Resolution[?],
+    transmittable16: Resolution[?], transmittable17: Resolution[?],
+    transmittable18: Resolution[?], transmittable19: Resolution[?],
+    transmittable20: Resolution[?], transmittable21: Resolution[?])
+: Container {
+    type Delegates =
+      transmittable0.Type / transmittable1.Type /
+      transmittable2.Type / transmittable3.Type /
+      transmittable4.Type / transmittable5.Type /
+      transmittable6.Type / transmittable7.Type /
+      transmittable8.Type / transmittable9.Type /
+      transmittable10.Type / transmittable11.Type /
+      transmittable12.Type / transmittable13.Type /
+      transmittable14.Type / transmittable15.Type /
+      transmittable16.Type / transmittable17.Type /
+      transmittable18.Type / transmittable19.Type /
+      transmittable20.Type / transmittable21.Type
+  } = ???
+
+def test22 =
+  Resolution(
+    element22(
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0),
+      Resolution(element0), Resolution(element0)))