Balance And/Or types when forming lubs and glbs

Fixes scala#12915

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

@@ -1571,6 +1571,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  @see [[sufficientEither]] for the normal case
    */
   protected def either(op1: => Boolean, op2: => Boolean): Boolean =
+    Stats.record("TypeComparer.either")
     if ctx.mode.is(Mode.GadtConstraintInference) || useNecessaryEither then
       necessaryEither(op1, op2)
     else
@@ -2238,7 +2239,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  opportunistically merged.
    */
   final def andType(tp1: Type, tp2: Type, isErased: Boolean = ctx.erasedTypes): Type =
-    andTypeGen(tp1, tp2, AndType(_, _), isErased = isErased)
+    andTypeGen(tp1, tp2, AndType.balanced(_, _), isErased = isErased)
 
   final def simplifyAndTypeWithFallback(tp1: Type, tp2: Type, fallback: Type): Type =
     andTypeGen(tp1, tp2, (_, _) => fallback)
@@ -2260,7 +2261,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       val t2 = distributeOr(tp2, tp1, isSoft)
       if (t2.exists) t2
       else if (isErased) erasedLub(tp1, tp2)
-      else liftIfHK(tp1, tp2, OrType(_, _, soft = isSoft), _ | _, _ & _)
+      else liftIfHK(tp1, tp2, OrType.balanced(_, _, soft = isSoft), _ | _, _ & _)
     }
   }
 

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

@@ -1716,6 +1716,14 @@ object Types {
     /** If this is a proto type, WildcardType, otherwise the type itself */
     def dropIfProto: Type = this
 
+    /** If this is an AndType, the number of factors, 1 for all other types */
+    def andFactorCount: Int = 1
+
+    /** If this is a OrType, the number of factors if that match `soft`,
+     *  1 for all other types.
+     */
+    def orFactorCount(soft: Boolean): Int = 1
+
 // ----- Substitutions -----------------------------------------------------
 
     /** Substitute all types that refer in their symbol attribute to
@@ -3110,6 +3118,12 @@ object Types {
       myBaseClasses
     }
 
+    private var myFactorCount = 0
+    override def andFactorCount =
+      if myFactorCount == 0 then
+      	myFactorCount = tp1.andFactorCount + tp2.andFactorCount
+      myFactorCount
+
     def derivedAndType(tp1: Type, tp2: Type)(using Context): Type =
       if ((tp1 eq this.tp1) && (tp2 eq this.tp2)) this
       else AndType.make(tp1, tp2, checkValid = true)
@@ -3135,6 +3149,23 @@ object Types {
       unchecked(tp1, tp2)
     }
 
+    def balanced(tp1: Type, tp2: Type)(using Context): AndType =
+      tp1 match
+        case AndType(tp11, tp12) if tp1.andFactorCount > tp2.andFactorCount * 2 =>
+          if tp11.andFactorCount < tp12.andFactorCount then
+            return apply(tp12, balanced(tp11, tp2))
+          else
+            return apply(tp11, balanced(tp12, tp2))
+        case _ =>
+      tp2 match
+        case AndType(tp21, tp22) if tp2.andFactorCount > tp1.andFactorCount * 2 =>
+          if tp22.andFactorCount < tp21.andFactorCount then
+            return apply(balanced(tp1, tp22), tp21)
+          else
+            return apply(balanced(tp1, tp21), tp22)
+        case _ =>
+      apply(tp1, tp2)
+
     def unchecked(tp1: Type, tp2: Type)(using Context): AndType = {
       assertUnerased()
       unique(new CachedAndType(tp1, tp2))
@@ -3181,6 +3212,14 @@ object Types {
       myBaseClasses
     }
 
+    private var myFactorCount = 0
+    override def orFactorCount(soft: Boolean) =
+      if this.isSoft == soft then
+        if myFactorCount == 0 then
+          myFactorCount = tp1.orFactorCount(soft) + tp2.orFactorCount(soft)
+        myFactorCount
+      else 1
+
     assert(tp1.isValueTypeOrWildcard &&
            tp2.isValueTypeOrWildcard, s"$tp1 $tp2")
 
@@ -3238,10 +3277,29 @@ object Types {
   final class CachedOrType(tp1: Type, tp2: Type, override val isSoft: Boolean) extends OrType(tp1, tp2)
 
   object OrType {
+
     def apply(tp1: Type, tp2: Type, soft: Boolean)(using Context): OrType = {
       assertUnerased()
       unique(new CachedOrType(tp1, tp2, soft))
     }
+
+    def balanced(tp1: Type, tp2: Type, soft: Boolean)(using Context): OrType =
+      tp1 match
+        case OrType(tp11, tp12) if tp1.orFactorCount(soft) > tp2.orFactorCount(soft) * 2 =>
+          if tp11.orFactorCount(soft) < tp12.orFactorCount(soft) then
+            return apply(tp12, balanced(tp11, tp2, soft), soft)
+          else
+            return apply(tp11, balanced(tp12, tp2, soft), soft)
+        case _ =>
+      tp2 match
+        case OrType(tp21, tp22) if tp2.orFactorCount(soft) > tp1.orFactorCount(soft) * 2 =>
+          if tp22.orFactorCount(soft) < tp21.orFactorCount(soft) then
+            return apply(balanced(tp1, tp22, soft), tp21, soft)
+          else
+            return apply(balanced(tp1, tp21, soft), tp22, soft)
+        case _ =>
+      apply(tp1, tp2, soft)
+
     def make(tp1: Type, tp2: Type, soft: Boolean)(using Context): Type =
       if (tp1 eq tp2) tp1
       else apply(tp1, tp2, soft)

tests/pos/i12915.scala

@@ -0,0 +1,105 @@
+trait E[T]
+
+class X {
+  val e1: E[Int] = ???
+  val e2: E[String] = ???
+  val e3: E[List[Int]] = ???
+  val e4: E[List[String]] = ???
+  val e5: E[Double] = ???
+  val e6: E[(String, String)] = ???
+  val e7: E[(String, Int)] = ???
+  val e8: E[(Int, List[String])] = ???
+  val e9: E[Long] = ???
+  val e10: E[(Long, Long)] = ???
+  val e11: E[(Long, Long, Int)] = ???
+  val e12: E[List[Long]] = ???
+  val e13: E[List[Int]] = ???
+  val e14: E[(String, String)] = ???
+  val e15: E[(String, String, String)] = ???
+  val e16: E[(Int, String)] = ???
+  val e17: E[(String, Long, String)] = ???
+  val e18: E[(Long, String, String)] = ???
+  val e19: E[(String, String, Long)] = ???
+  val e20: E[(String, Int, String)] = ???
+  val e21: E[(Int, String, String)] = ???
+  val e22: E[(String, String, Int)] = ???
+  val e23: E[(String, String, Boolean)] = ???
+  val e24: E[(Boolean, Boolean, String)] = ???
+  val e25: E[(String, Int, Boolean)] = ???
+  val e26: E[List[(String, String)]] = ???
+  val e27: E[List[(Int, String)]] = ???
+  val e28: E[List[(String, Int)]] = ???
+  val e29: E[List[(Long, String)]] = ???
+  val e30: E[List[(String, Long)]] = ???
+  val e31: E[List[(Boolean, String)]] = ???
+  val e32: E[List[(String, Boolean)]] = ???
+  val e33: E[List[((String, String), String)]] = ???
+  val e34: E[List[((String, Int), String)]] = ???
+  val e35: E[List[((Long, String), String)]] = ???
+  val e36: E[List[((Boolean, String), String)]] = ???
+  val e37: E[List[((String, String), Int)]] = ???
+  val e38: E[List[((String, String), (String, Int))]] = ???
+  val e39: E[List[((Boolean, Long), (String, Int))]] = ???
+  val e40: E[List[((Int, Long), (Boolean, Int))]] = ???
+  val e41: E[List[((String, (Int, String)), (String, Int))]] = ???
+  val e42: E[List[((Boolean, (Int, String)), (String, Int))]] = ???
+  val e43: E[List[((String, (Int, String)), (Boolean, Int))]] = ???
+  val e44: E[(Int, List[String], Long)] = ???
+  val e45: E[(Int, List[Int], Long)] = ???
+  val e46: E[(Int, List[Long], Long)] = ???
+  val e47: E[(String, List[String], Long)] = ???
+  val e48: E[(Int, List[String], Boolean)] = ???
+  val e49: E[Char] = ???
+
+  val all = List(
+    e1,
+    e2,
+    e3,
+    e4,
+    e5,
+    e6,
+    e7,
+    e8,
+    e9,
+    e10,
+    e11,
+    e12,
+    e13,
+    e14,
+    e15,
+    e16,
+    e17,
+    e18,
+    e19,
+    e20,
+    e21,
+    e22,
+    e23,
+    e24,
+    e25,
+    e26,
+    e27,
+    e28,
+    e29,
+    e30,
+    e31,
+    e32,
+    e33,
+    e34,
+    e35,
+    e36,
+    e37,
+    e38,
+    e39,
+    e40,
+    e41,
+    e42,
+    e43,
+    e44,
+    e45,
+    e46,
+    e47,
+    e48,
+    e49
+  )
+}