Improve partial unification handling

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

@@ -849,6 +849,85 @@ class TypeComparer(using val comparerCtx: Context) extends ConstraintHandling wi
         case _ =>
       isSubType(pre1, pre2)
 
+    /** Compare `tycon[args]` with `other := otherTycon[otherArgs]`, via `>:>` if fromBelow is true, `<:<` otherwise
+     *  (we call this relationship `~:~` in the rest of this comment).
+     *
+     *  This method works by:
+     *
+     *  1. Choosing an appropriate type constructor `adaptedTycon`
+     *  2. Constraining `tycon` such that `tycon ~:~ adaptedTycon`
+     *  3. Recursing on `adaptedTycon[args] ~:~ other`
+     *
+     *  So, how do we pick `adaptedTycon`? When `args` and `otherArgs` have the
+     *  same length the answer is simply:
+     *
+     *    adaptedTycon := otherTycon
+     *
+     *  But we also handle having `args.length < otherArgs.length`, in which
+     *  case we need to make up a type constructor of the right kind. For
+     *  example, if `fromBelow = false` and we're comparing:
+     *
+     *    ?F[A] <:< Either[String, B] where `?F <: [X] =>> Any`
+     *
+     *  we will choose:
+     *
+     *    adaptedTycon := [X] =>> Either[String, X]
+     *
+     *  this allows us to constrain:
+     *
+     *    ?F <: adaptedTycon
+     *
+     *  and then recurse on:
+     *
+     *    adaptedTycon[A] <:< Either[String, B]
+     *
+     *  In general, given:
+     *
+     *  - k := args.length
+     *  - d := otherArgs.length - k
+     *
+     *  `adaptedTycon` will be:
+     *
+     *    [T_0, ..., T_k-1] =>> otherTycon[otherArgs(0), ..., otherArgs(d-1), T_0, ..., T_k-1]
+     *
+     *  where `T_n` has the same bounds as `otherTycon.typeParams(d+n)`
+     *
+     *  Historical note: this strategy is known in Scala as "partial unification"
+     *  (even though the type constructor variable isn't actually unified but only
+     *  has one of its bounds constrained), for background see:
+     *  - The infamous SI-2712: https://github.com/scala/bug/issues/2712
+     *  - The PR against Scala 2.12 implementing -Ypartial-unification: https://github.com/scala/scala/pull/5102
+     *  - Some explanations on how this impacts API design: https://gist.github.com/djspiewak/7a81a395c461fd3a09a6941d4cd040f2
+     */
+    def compareAppliedTypeParamRef(tycon: TypeParamRef, args: List[Type], other: AppliedType, fromBelow: Boolean): Boolean =
+      def directionalIsSubType(tp1: Type, tp2: Type): Boolean =
+        if fromBelow then isSubType(tp2, tp1) else isSubType(tp1, tp2)
+      def directionalRecur(tp1: Type, tp2: Type): Boolean =
+        if fromBelow then recur(tp2, tp1) else recur(tp1, tp2)
+
+      val otherTycon = other.tycon
+      val otherArgs = other.args
+
+      val d = otherArgs.length - args.length
+      d >= 0 && {
+        val tparams = tycon.typeParams
+        val remainingTparams = otherTycon.typeParams.drop(d)
+        variancesConform(remainingTparams, tparams) && {
+          val adaptedTycon =
+            if d > 0 then
+              HKTypeLambda(remainingTparams.map(_.paramName))(
+                tl => remainingTparams.map(remainingTparam =>
+                  tl.integrate(remainingTparams, remainingTparam.paramInfo).bounds),
+                tl => otherTycon.appliedTo(
+                  otherArgs.take(d) ++ tl.paramRefs))
+            else
+              otherTycon
+          (assumedTrue(tycon) || directionalIsSubType(tycon, adaptedTycon.ensureLambdaSub)) &&
+          directionalRecur(adaptedTycon.appliedTo(args), other)
+        }
+      }
+    end compareAppliedTypeParamRef
+
     /** Subtype test for the hk application `tp2 = tycon2[args2]`.
      */
     def compareAppliedType2(tp2: AppliedType, tycon2: Type, args2: List[Type]): Boolean = {
@@ -860,13 +939,35 @@ class TypeComparer(using val comparerCtx: Context) extends ConstraintHandling wi
        */
       def isMatchingApply(tp1: Type): Boolean = tp1 match {
         case AppliedType(tycon1, args1) =>
-          def loop(tycon1: Type, args1: List[Type]): Boolean = tycon1.dealiasKeepRefiningAnnots match {
+          // We intentionally do not dealias `tycon1` or `tycon2` here.
+          // `TypeApplications#appliedTo` already takes care of dealiasing type
+          // constructors when this can be done without affecting type
+          // inference, doing it here would not only prevent code from compiling
+          // but could also result in the wrong thing being inferred later, for example
+          // in `tests/run/hk-alias-unification.scala` we end up checking:
+          //
+          //   Foo[?F, ?T] <:< Foo[[X] =>> (X, String), Int]
+          //
+          // Naturally, we'd like to infer:
+          //
+          //   ?F := [X] => (X, String)
+          //
+          // but if we dealias `Foo` then we'll end up trying to check:
+          //
+          //   ErasedFoo[?F[?T]] <:< ErasedFoo[(Int, String)]
+          //
+          // Because of partial unification, this will succeed, but will produce the constraint:
+          //
+          //   ?F := [X] =>> (Int, X)
+          //
+          // Which is not what we wanted!
+          def loop(tycon1: Type, args1: List[Type]): Boolean = tycon1 match {
             case tycon1: TypeParamRef =>
               (tycon1 == tycon2 ||
                canConstrain(tycon1) && isSubType(tycon1, tycon2)) &&
               isSubArgs(args1, args2, tp1, tparams)
             case tycon1: TypeRef =>
-              tycon2.dealiasKeepRefiningAnnots match {
+              tycon2 match {
                 case tycon2: TypeRef =>
                   val tycon1sym = tycon1.symbol
                   val tycon2sym = tycon2.symbol
@@ -926,60 +1027,26 @@ class TypeComparer(using val comparerCtx: Context) extends ConstraintHandling wi
 
       /** `param2` can be instantiated to a type application prefix of the LHS
        *  or to a type application prefix of one of the LHS base class instances
-       *  and the resulting type application is a supertype of `tp1`,
-       *  or fallback to fourthTry.
+       *  and the resulting type application is a supertype of `tp1`.
        */
       def canInstantiate(tycon2: TypeParamRef): Boolean = {
-
-        /** Let
-         *
-         *    `tparams_1, ..., tparams_k-1`    be the type parameters of the rhs
-         *    `tparams1_1, ..., tparams1_n-1`  be the type parameters of the constructor of the lhs
-         *    `args1_1, ..., args1_n-1`        be the type arguments of the lhs
-         *    `d  =  n - k`
-         *
-         *  Returns `true` iff `d >= 0` and `tycon2` can be instantiated to
-         *
-         *      [tparams1_d, ... tparams1_n-1] -> tycon1[args_1, ..., args_d-1, tparams_d, ... tparams_n-1]
-         *
-         *  such that the resulting type application is a supertype of `tp1`.
-         */
         def appOK(tp1base: Type) = tp1base match {
           case tp1base: AppliedType =>
-            var tycon1 = tp1base.tycon
-            val args1 = tp1base.args
-            val tparams1all = tycon1.typeParams
-            val lengthDiff = tparams1all.length - tparams.length
-            lengthDiff >= 0 && {
-              val tparams1 = tparams1all.drop(lengthDiff)
-              variancesConform(tparams1, tparams) && {
-                if (lengthDiff > 0)
-                  tycon1 = HKTypeLambda(tparams1.map(_.paramName))(
-                    tl => tparams1.map(tparam => tl.integrate(tparams, tparam.paramInfo).bounds),
-                    tl => tp1base.tycon.appliedTo(args1.take(lengthDiff) ++
-                            tparams1.indices.toList.map(tl.paramRefs(_))))
-                (assumedTrue(tycon2) || isSubType(tycon1.ensureLambdaSub, tycon2)) &&
-                recur(tp1, tycon1.appliedTo(args2))
-              }
-            }
+            compareAppliedTypeParamRef(tycon2, args2, tp1base, fromBelow = true)
           case _ => false
         }
 
-        tp1.widen match {
-          case tp1w: AppliedType => appOK(tp1w)
-          case tp1w =>
-            tp1w.typeSymbol.isClass && {
-              val classBounds = tycon2.classSymbols
-              def liftToBase(bcs: List[ClassSymbol]): Boolean = bcs match {
-                case bc :: bcs1 =>
-                  classBounds.exists(bc.derivesFrom) && appOK(nonExprBaseType(tp1, bc))
-                  || liftToBase(bcs1)
-                case _ =>
-                  false
-              }
-              liftToBase(tp1w.baseClasses)
-            } ||
-            fourthTry
+        val tp1w = tp1.widen
+        appOK(tp1w) || tp1w.typeSymbol.isClass && {
+          val classBounds = tycon2.classSymbols
+          def liftToBase(bcs: List[ClassSymbol]): Boolean = bcs match {
+            case bc :: bcs1 =>
+              classBounds.exists(bc.derivesFrom) && appOK(nonExprBaseType(tp1, bc))
+              || liftToBase(bcs1)
+            case _ =>
+              false
+          }
+          liftToBase(tp1w.baseClasses)
         }
       }
 
@@ -1043,8 +1110,8 @@ class TypeComparer(using val comparerCtx: Context) extends ConstraintHandling wi
       tycon1 match {
         case param1: TypeParamRef =>
           def canInstantiate = tp2 match {
-            case AppliedType(tycon2, args2) =>
-              isSubType(param1, tycon2.ensureLambdaSub) && isSubArgs(args1, args2, tp1, tycon2.typeParams)
+            case tp2base: AppliedType =>
+              compareAppliedTypeParamRef(param1, args1, tp2base, fromBelow = false)
             case _ =>
               false
           }

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -15,6 +15,7 @@ import scala.io.Source
 import scala.util.{Random, Try, Failure => TryFailure, Success => TrySuccess, Using}
 import scala.util.control.NonFatal
 import scala.util.matching.Regex
+import scala.collection.mutable.ListBuffer
 
 import dotc.{Compiler, Driver}
 import dotc.core.Contexts._
@@ -543,11 +544,18 @@ trait ParallelTesting extends RunnerOrchestration { self =>
         }
 
         pool.shutdown()
+
         if (!pool.awaitTermination(20, TimeUnit.MINUTES)) {
+          val remaining = new ListBuffer[TestSource]
+          filteredSources.lazyZip(eventualResults).foreach { (src, res) =>
+            if (!res.isDone)
+              remaining += src
+          }
+
           pool.shutdownNow()
           System.setOut(realStdout)
           System.setErr(realStderr)
-          throw new TimeoutException("Compiling targets timed out")
+          throw new TimeoutException(s"Compiling targets timed out, remaining targets: ${remaining.mkString(", ")}")
         }
 
         eventualResults.foreach { x =>

tests/neg/i3452.scala

@@ -6,8 +6,22 @@ object Test {
   implicit def case1[F[_]](implicit t: => TC[F[Any]]): TC[Tuple2K[[_] =>> Any, F, Any]] = ???
   implicit def case2[A, F[_]](implicit r: TC[F[Any]]): TC[A] = ???
 
+  // Disabled because it leads to an infinite loop in implicit search
+  // this is probably the same issue as https://github.com/lampepfl/dotty/issues/9568
+  // implicitly[TC[Int]] // was: error
+}
+
+object Test1 {
+  case class Tuple2K[H[_], T[_], X](h: H[X], t: T[X])
+
+  trait TC[A]
+
+  implicit def case1[F[_]](implicit t: TC[F[Any]]): TC[Tuple2K[[_] =>> Any, F, Any]] = ???
+  implicit def case2[A, F[_]](implicit r: TC[F[Any]]): TC[A] = ???
+
   implicitly[TC[Int]] // error
 }
+
 object Test2 {
   trait TC[A]
 

tests/neg/t2712-8.scala

@@ -0,0 +1,10 @@
+object Test extends App {
+  class L[A]
+  class Quux0[B, CC[_]]
+  class Quux[C] extends Quux0[C, L]
+
+  def foo[D[_]](x: D[D[Boolean]]) = ???
+  def bar: Quux[Int] = ???
+
+  foo(bar) // error: Found: Test.Quux[Int]  Required: D[D[Boolean]]
+}

tests/pos/anykind.scala

@@ -56,11 +56,11 @@ object Test {
   object Kinder extends KinderLowerImplicits {
     type Aux[MA, M0 <: AnyKind, Args0 <: HList] = Kinder[MA] { type M = M0; type Args = Args0 }
 
-    implicit def kinder2[M0[_, _], A0, B0]: Kinder.Aux[M0[A0, B0], M0, A0 :: B0 :: HNil] = new Kinder[M0[A0, B0]] { type M[t, u] = M0[t, u]; type Args = A0 :: B0 :: HNil }
     implicit def kinder1[M0[_], A0]: Kinder.Aux[M0[A0], M0, A0 :: HNil] = new Kinder[M0[A0]] { type M[t] = M0[t]; type Args = A0 :: HNil }
   }
 
   trait KinderLowerImplicits {
+    implicit def kinder2[M0[_, _], A0, B0]: Kinder.Aux[M0[A0, B0], M0, A0 :: B0 :: HNil] = new Kinder[M0[A0, B0]] { type M[t, u] = M0[t, u]; type Args = A0 :: B0 :: HNil }
     implicit def kinder0[A]: Kinder.Aux[A, A, HNil] = new Kinder[A] { type M = A; type Args = HNil }
   }
 

tests/pos/i6565.scala

@@ -8,10 +8,10 @@ extension [O, U](o: Lifted[O]) def flatMap(f: O => Lifted[U]): Lifted[U] = ???
 
 val error: Err = Err()
 
-lazy val ok: Lifted[String] = { // ok despite map returning a union
-  point("a").map(_ => if true then "foo" else error) // ok
+lazy val ok: Lifted[String] = {
+  point("a").flatMap(_ => if true then "foo" else error)
 }
 
 lazy val nowAlsoOK: Lifted[String] = {
-  point("a").flatMap(_ => point("b").map(_ => if true then "foo" else error))
+  point("a").flatMap(_ => point("b").flatMap(_ => if true then "foo" else error))
 }

tests/pos/i9478.scala

@@ -0,0 +1,6 @@
+class Foo[T[_, _], F[_], A, B](val fa: T[F[A], F[B]])
+
+object Test {
+  def x[T[_, _]](tmab: T[Either[Int, String], Either[Int, Int]]) =
+    new Foo(tmab)
+}

tests/pos/t2712-2b.scala

@@ -0,0 +1,18 @@
+package test
+
+class X1
+class X2
+class X3
+
+trait One[A]
+trait Two[A, B]
+
+class Foo extends Two[X1, X2] with One[X3]
+object Test {
+  def test1[M[_], A](x: M[A]): M[A] = x
+
+  val foo = new Foo
+
+  test1(foo): One[X3] // fails in Scala 2 with partial unification enabled, works in Dotty
+  test1(foo): Two[X1, X2]
+}

tests/pos/t2712-8.scala

@@ -0,0 +1,9 @@
+class Two[A, B]
+class One[A] extends Two[A, A]
+
+object Test {
+  def foo[F[_, _]](x: F[Int, Int]) = x
+
+  val t: One[Int] = ???
+  foo(t)
+}

tests/run/hk-alias-unification.scala

@@ -0,0 +1,23 @@
+trait Bla[T]
+object Bla {
+  implicit def blaInt: Bla[Int] = new Bla[Int] {}
+  implicit def blaString: Bla[String] = new Bla[String] {
+    assert(false, "I should not be summoned!")
+  }
+}
+
+trait ErasedFoo[FT]
+object Test {
+  type Foo[F[_], T] = ErasedFoo[F[T]]
+  type Foo2[F[_], T] = Foo[F, T]
+
+  def mkFoo[F[_], T](implicit gen: Bla[T]): Foo[F, T] = new Foo[F, T] {}
+  def mkFoo2[F[_], T](implicit gen: Bla[T]): Foo2[F, T] = new Foo2[F, T] {}
+
+  def main(args: Array[String]): Unit = {
+    val a: Foo[[X] =>> (X, String), Int] = mkFoo
+    val b: Foo2[[X] =>> (X, String), Int] = mkFoo
+    val c: Foo[[X] =>> (X, String), Int] = mkFoo2
+  }
+}
+