Higher-kinded type variable unification.

Can cause ambiguous implicits, so is under a compiler flag
-Yhk-typevar-unification.

Fixes scala/bug#10197
Fixes scala/bug#10213
Fixes scala/bug#10238
Fixes scala/bug#10372
Presents an alternative fix to scala/bug#6895.

Author: TomasMikula

project/ScalaOptionParser.scala

@@ -85,7 +85,7 @@ object ScalaOptionParser {
   private def booleanSettingNames = List("-X", "-Xcheckinit", "-Xdev", "-Xdisable-assertions", "-Xexperimental", "-Xfatal-warnings", "-Xfull-lubs", "-Xfuture", "-Xlog-free-terms", "-Xlog-free-types", "-Xlog-implicit-conversions", "-Xlog-implicits", "-Xlog-reflective-calls",
     "-Xno-forwarders", "-Xno-patmat-analysis", "-Xno-uescape", "-Xnojline", "-Xprint-pos", "-Xprint-types", "-Xprompt", "-Xresident", "-Xshow-phases", "-Xstrict-inference", "-Xverify", "-Y",
     "-Ybreak-cycles", "-Ydebug", "-Ycompact-trees", "-YdisableFlatCpCaching", "-Ydoc-debug",
-    "-Yide-debug", "-Yinfer-argument-types",
+    "-Yhk-typevar-unification", "-Yide-debug", "-Yinfer-argument-types",
     "-Yissue-debug", "-Ylog-classpath", "-Ymacro-debug-lite", "-Ymacro-debug-verbose", "-Ymacro-no-expand",
     "-Yno-completion", "-Yno-generic-signatures", "-Yno-imports", "-Yno-predef",
     "-Yoverride-objects", "-Yoverride-vars", "-Ypatmat-debug", "-Yno-adapted-args", "-Ypartial-unification", "-Ypos-debug", "-Ypresentation-debug",

src/compiler/scala/tools/nsc/settings/ScalaSettings.scala

@@ -30,7 +30,7 @@ trait ScalaSettings extends AbsScalaSettings
   protected def defaultClasspath = sys.env.getOrElse("CLASSPATH", ".")
 
   /** Enabled under -Xexperimental. */
-  protected def experimentalSettings = List[BooleanSetting](YpartialUnification)
+  protected def experimentalSettings = List[BooleanSetting](YhkTypevarUnification, YpartialUnification)
 
   /** Enabled under -Xfuture. */
   protected def futureSettings = List[BooleanSetting]()
@@ -220,6 +220,7 @@ trait ScalaSettings extends AbsScalaSettings
   val YmethodInfer    = BooleanSetting    ("-Yinfer-argument-types", "Infer types for arguments of overridden methods.")
   val YdisableFlatCpCaching  = BooleanSetting    ("-YdisableFlatCpCaching", "Do not cache flat classpath representation of classpath elements from jars across compiler instances.")
   val YpartialUnification = BooleanSetting ("-Ypartial-unification", "Enable partial unification in type constructor inference")
+  val YhkTypevarUnification = BooleanSetting ("-Yhk-typevar-unification", "Enable unification of higher-kinded type variables with type constructors")
   val Yvirtpatmat     = BooleanSetting    ("-Yvirtpatmat", "Enable pattern matcher virtualization")
 
   val exposeEmptyPackage = BooleanSetting ("-Yexpose-empty-package", "Internal only: expose the empty package.").internalOnly()

src/reflect/mima-filters/2.12.0.forwards.excludes

@@ -13,4 +13,5 @@ ProblemFilters.exclude[MissingClassProblem]("scala.reflect.io.FileZipArchive$Laz
 ProblemFilters.exclude[DirectMissingMethodProblem]("scala.reflect.io.ZipArchive.closeZipFile")
 ProblemFilters.exclude[MissingClassProblem]("scala.reflect.io.FileZipArchive$LeakyEntry")
 
-ProblemFilters.exclude[DirectMissingMethodProblem]("scala.reflect.runtime.SynchronizedSymbols#SynchronizedSymbol.exists")
+ProblemFilters.exclude[DirectMissingMethodProblem]("scala.reflect.runtime.SynchronizedSymbols#SynchronizedSymbol.exists")
+ProblemFilters.exclude[DirectMissingMethodProblem]("scala.reflect.runtime.Settings.YhkTypevarUnification")

src/reflect/scala/reflect/internal/Types.scala

@@ -3325,7 +3325,7 @@ trait Types
     )
     override def etaExpand: Type = (
       if (!isHigherKinded) this
-      else logResult("Normalizing HK $this")(typeFun(params, applyArgs(params map (_.typeConstructor))))
+      else logResult(s"Normalizing HK $this")(typeFun(params, applyArgs(params map (_.typeConstructor))))
     )
     override def typeSymbol = origin.typeSymbol
 

src/reflect/scala/reflect/internal/settings/MutableSettings.scala

@@ -53,6 +53,7 @@ abstract class MutableSettings extends AbsSettings {
   def printtypes: BooleanSetting
   def uniqid: BooleanSetting
   def verbose: BooleanSetting
+  def YhkTypevarUnification: BooleanSetting
   def YpartialUnification: BooleanSetting
   def Yvirtpatmat: BooleanSetting
 

src/reflect/scala/reflect/internal/tpe/TypeComparers.scala

@@ -365,7 +365,32 @@ trait TypeComparers {
 
   // @assume tp1.isHigherKinded || tp2.isHigherKinded
   def isHKSubType(tp1: Type, tp2: Type, depth: Depth): Boolean = {
-    def isSub(ntp1: Type, ntp2: Type) = (ntp1.withoutAnnotations, ntp2.withoutAnnotations) match {
+
+    def isSubHKTypeVar(tp1: Type, tp2: Type) = (tp1, tp2) match {
+      case (tv1 @ TypeVar(_, _), tv2 @ TypeVar(_, _)) =>
+        reporter.warning(tv1.typeSymbol.pos,
+          sm"""|compiler bug: Unexpected code path: testing two type variables for subtype relation:
+               |  ${tv1} <:< ${tv2}
+               |Please report bug at https://github.com/scala/bug/issues
+            """.trim)
+        false
+      case (tp1, tv2 @ TypeVar(_, _)) =>
+        val ntp1 = tp1.normalize
+        (tv2.params corresponds ntp1.typeParams)(methodHigherOrderTypeParamsSubVariance) &&
+        { tv2.addLoBound(ntp1); true }
+      case (tv1 @ TypeVar(_, _), tp2) =>
+        val ntp2 = tp2.normalize
+        (ntp2.typeParams corresponds tv1.params)(methodHigherOrderTypeParamsSubVariance) &&
+        { tv1.addHiBound(ntp2); true }
+      case _ =>
+        false
+    }
+
+    def isSub(tp1: Type, tp2: Type) =
+      settings.YhkTypevarUnification && isSubHKTypeVar(tp1, tp2) ||
+        isSub2(tp1.normalize, tp2.normalize)  // @M! normalize reduces higher-kinded case to PolyType's
+
+    def isSub2(ntp1: Type, ntp2: Type) = (ntp1, ntp2) match {
       case (TypeRef(_, AnyClass, _), _)                                     => false                    // avoid some warnings when Nothing/Any are on the other side
       case (_, TypeRef(_, NothingClass, _))                                 => false
       case (pt1: PolyType, pt2: PolyType)                                   => isPolySubType(pt1, pt2)  // @assume both .isHigherKinded (both normalized to PolyType)
@@ -381,7 +406,7 @@ trait TypeComparers {
       || (if (isNoArgStaticClassTypeRef(tp1) && isNoArgStaticClassTypeRef(tp2))
             tp1.typeSymbolDirect.isNonBottomSubClass(tp2.typeSymbolDirect) // OPT faster than comparing eta-expanded types
           else
-            isSub(tp1.normalize, tp2.normalize) && annotationsConform(tp1, tp2)  // @M! normalize reduces higher-kinded case to PolyType's
+            isSub(tp1.withoutAnnotations, tp2.withoutAnnotations) && annotationsConform(tp1, tp2)
          )
     )
   }

src/reflect/scala/reflect/runtime/Settings.scala

@@ -47,6 +47,7 @@ private[reflect] class Settings extends MutableSettings {
   val printtypes        = new BooleanSetting(false)
   val uniqid            = new BooleanSetting(false)
   val verbose           = new BooleanSetting(false)
+  val YhkTypevarUnification = new BooleanSetting(false)
   val YpartialUnification = new BooleanSetting(false)
   val Yvirtpatmat       = new BooleanSetting(false)
 

test/files/neg/hk-typevar-unification.check

@@ -0,0 +1,11 @@
+hk-typevar-unification.scala:16: error: inferred kinds of the type arguments ([_]Foo[_]) do not conform to the expected kinds of the type parameters (type F).
+[_]Foo[_]'s type parameters do not match type F's expected parameters:
+type _ (in class Foo) is invariant, but type _ is declared covariant
+  g(tcFoo)
+  ^
+hk-typevar-unification.scala:16: error: type mismatch;
+ found   : TC[Foo]
+ required: TC[F]
+  g(tcFoo)
+    ^
+two errors found

test/files/neg/hk-typevar-unification.flags

@@ -0,0 +1 @@
+-Yhk-typevar-unification

test/files/neg/hk-typevar-unification.scala

@@ -0,0 +1,17 @@
+trait TC[F[_]]
+class A
+class Foo[_]
+
+object Test {
+
+  def f[F[_ <: A]](tc: TC[F]): Unit = ()
+  def g[F[+_]]    (tc: TC[F]): Unit = ()
+
+  val tcFoo: TC[Foo] = new TC[Foo] {}
+
+  // incompatible bounds
+  f(tcFoo)
+
+  // incompatible variance
+  g(tcFoo)
+}

test/files/pos/t10197.flags

@@ -0,0 +1 @@
+-Yhk-typevar-unification

test/files/pos/t10197.scala

@@ -0,0 +1,38 @@
+import scala.language.higherKinds
+
+final case class Getter[S, A](get: S => A)
+
+final case class Wrap[F[_], A](value: F[A])
+
+object Wrap {
+  // Helper to defer specifying second argument to Wrap.
+  // Basically a type lambda specialized for Wrap.
+  // Wr[F]#ap[A] =:= Wrap[F, A]
+  type Wr[F[_]] = { type ap[A] = Wrap[F, A] }
+
+  implicit def unwrapper[F[_], A]: Getter[Wrap[F, A], F[A]] =
+    Getter(w => w.value)
+}
+
+object Test {
+  import Wrap._
+
+  type Foo[A] = List[A]
+  type Bar[A] = String
+
+  type WrapFoo1[A] = Wrap[Foo, A]
+  type WrapBar1[A] = Wrap[Bar, A]
+
+  implicitly[Getter[WrapFoo1[Int], Foo[Int]]]
+  implicitly[Getter[WrapBar1[Int], Bar[Int]]]
+
+  type WrapFoo2[A] = Wr[Foo]#ap[A]
+  type WrapBar2[A] = Wr[Bar]#ap[A]
+
+  // here's evidence that the new types are the same as the old ones
+  implicitly[WrapFoo2[Int] =:= WrapFoo1[Int]]
+  implicitly[WrapBar2[Int] =:= WrapBar1[Int]]
+
+  implicitly[Getter[WrapFoo2[Int], Foo[Int]]]
+  implicitly[Getter[WrapBar2[Int], Bar[Int]]]
+}

test/files/pos/t10213.flags

@@ -0,0 +1 @@
+-Yhk-typevar-unification

test/files/pos/t10213.scala

@@ -0,0 +1,53 @@
+import scala.language.higherKinds
+
+final case class Coproduct[F[_], G[_], A](run: Either[F[A], G[A]])
+
+object Coproduct {
+
+  sealed trait Builder {
+    type Out[_]
+  }
+
+  sealed trait :++:[F[_], G[_]] extends Builder {
+    type Out[A] = Coproduct[F, G, A]
+  }
+
+  sealed trait :+:[F[_], B <: Builder] extends Builder {
+    type Out[A] = Coproduct[F, B#Out, A]
+  }
+}
+
+trait Inject[F[_], H[_]] {
+  def inj[A](fa: F[A]): H[A]
+}
+
+object Inject {
+  import Coproduct._
+
+  implicit def reflexiveInject[F[_]]: Inject[F, F] =
+    new Inject[F, F] {
+      def inj[A](fa: F[A]): F[A] = fa
+    }
+
+  implicit def injectLeft[F[_], G[_]]: Inject[F, (F :++: G)#Out] =
+    new Inject[F, (F :++: G)#Out] {
+      def inj[A](fa: F[A]): Coproduct[F, G, A] = Coproduct(Left(fa))
+    }
+
+  implicit def injectRight[F[_], G[_], H[_]](implicit I: Inject[F, H]): Inject[F, (G :++: H)#Out] =
+    new Inject[F, (G :++: H)#Out] {
+      def inj[A](fa: F[A]): Coproduct[G, H , A] = Coproduct(Right(I.inj(fa)))
+    }
+}
+
+object Test1 {
+  import Coproduct.{:++:, :+:}
+
+  class Foo[A]
+  class Bar[A]
+  class Baz[A]
+
+  implicitly[Inject[Baz, (Foo :+: Bar :++: Baz)#Out]]
+
+  implicitly[Inject[Baz, ({ type Out[A] = Coproduct[Foo, ({ type Out1[a] = Coproduct[Bar, Baz, a] })#Out1, A] })#Out]]
+}

test/files/pos/t10238.flags

@@ -0,0 +1 @@
+-Yhk-typevar-unification

test/files/pos/t10238.scala

@@ -0,0 +1,36 @@
+object Test {
+
+  // Data types
+
+  type Id[A] = A
+
+  class MaybeT[F[_], A]
+
+  type Maybe[A] = MaybeT[Id, A]
+
+  type MaybeMaybe[A] = MaybeT[Maybe, A]
+
+
+  // Typeclass
+
+  trait Monad[F[_]]
+
+
+  // Instances
+
+  implicit val monadId: Monad[Id] = ???
+
+  implicit def monadMaybeT[F[_]: Monad]: Monad[({ type λ[A] = MaybeT[F, A] })#λ] = ???
+
+  implicit val monadOption: Monad[Option] = ???
+
+
+  // Implicit search tests
+
+  implicitly[Monad[Id]]
+  implicitly[Monad[({ type λ[A] = A })#λ]]
+  implicitly[Monad[Maybe]]
+  implicitly[Monad[({ type λ[A] = MaybeT[Id, A] })#λ]]
+  implicitly[Monad[MaybeMaybe]]
+  implicitly[Monad[({ type λ[A] = MaybeT[Maybe, A] })#λ]]
+}

test/files/pos/t10372.flags

@@ -0,0 +1 @@
+-Yhk-typevar-unification

test/files/pos/t10372.scala

@@ -0,0 +1,16 @@
+import scala.language.higherKinds
+import scala.language.implicitConversions
+
+object Test {
+  class Expected[T, Func[_]]
+  implicit def conv[T, Func[_]](i : Int) : Expected[T, Func] = ???
+  type FuncId[T] = T
+
+  object DoesNotCompile {
+    class Bla {
+      type Alias[T] = Expected[T, FuncId]
+      def bla[T](expected : Alias[T]) : Unit = {}
+    }
+    (new Bla).bla(2)
+  }
+}

test/files/pos/t6895b-2.flags

@@ -0,0 +1,2 @@
+-Yhk-typevar-unification
+

test/files/pos/t6895b-2.scala

@@ -0,0 +1,39 @@
+trait Foo[F[_]]
+trait Bar[F[_], A]
+
+trait Or[A, B]
+
+class Test {
+  implicit def orFoo[A]: Foo[({type L[X] = Or[A, X]})#L] = ???
+  implicit def barFoo[F[_]](implicit f: Foo[F]): Foo[({type L[X] = Bar[F, X]})#L] = ???
+
+  // Now we can define a couple of type aliases:
+  type StringOr[X] = Or[String, X]
+  type BarStringOr[X] = Bar[StringOr, X]
+
+  // ok
+  implicitly[Foo[BarStringOr]]
+  barFoo[StringOr](null) : Foo[BarStringOr]
+  barFoo(null) : Foo[BarStringOr]
+
+  // nok
+  implicitly[Foo[({type L[X] = Bar[StringOr, X]})#L]]
+  // Let's write the application explicitly, and then
+  // compile with just this line enabled and -explaintypes.
+  barFoo(null) : Foo[({type L[X] = Bar[StringOr, X]})#L]
+
+  // Foo[[X]Bar[F,X]] <: Foo[[X]Bar[[X]Or[String,X],X]]?
+  //   Bar[[X]Or[String,X],X] <: Bar[F,X]?
+  //     F[_] <: Or[String,_]?
+  //     false
+  //   false
+  // false
+
+  // Note that the type annotation above is typechecked as
+  // Foo[[X]Bar[[X]Or[String,X],X]], ie the type alias `L`
+  // is eta expanded.
+  //
+  // This is done so that it does not escape its defining scope.
+  // However, one this is done, higher kinded inference
+  // no longer is able to unify F with `StringOr` (scala/bug#2712)
+}

test/files/run/hk-typevar-unification.check

@@ -0,0 +1,8 @@
+Some(1)
+Some(1)
+Some((hi,5))
+Some((hi,5))
+Some(X)
+Some(X)
+Some(X)
+Some(X)

test/files/run/hk-typevar-unification.flags

@@ -0,0 +1 @@
+-Yhk-typevar-unification