Merge pull request scala#12924 from dotty-staging/fix-i12126

ProtoTypes#normalizedCompatible: keep more constraints

compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -64,19 +64,8 @@ object ProtoTypes {
                 i"""normalizedCompatible for $poly, $pt = $result
                    |constraint was: ${ctx.typerState.constraint}
                    |constraint now: ${newctx.typerState.constraint}""")
-            if result
-                && (ctx.typerState.constraint ne newctx.typerState.constraint)
-                && {
-                  val existingVars = ctx.typerState.uninstVars.toSet
-                  newctx.typerState.uninstVars.forall(existingVars.contains)
-                }
-            then newctx.typerState.commit()
-              // If the new constrait contains fresh type variables we cannot keep it,
-              // since those type variables are not instantiated anywhere in the source.
-              // See pos/i6682a.scala for a test case. See pos/11243.scala and pos/i5773b.scala
-              // for tests where it matters that we keep the constraint otherwise.
-              // TODO: A better solution would clean the new constraint, so that it "avoids"
-              // the problematic type variables. But we have not implemented such an algorithm yet.
+            if result && (ctx.typerState.constraint ne newctx.typerState.constraint) then
+              newctx.typerState.commit()
             result
           case _ => testCompat
       else explore(testCompat)

tests/pos/i12126.scala

@@ -0,0 +1,59 @@
+object Structures:
+
+  trait Functor[F[_]]:
+    extension [A](fa: F[A])
+      def map[B](f: A => B): F[B]
+      def as[B](b: B): F[B] = map(_ => b)
+      def void: F[Unit] = as(())
+
+  trait Applicative[F[_]] extends Functor[F]:
+    def pure[A](a: A): F[A]
+    def unit: F[Unit] = pure(())
+    extension[A](fa: F[A])
+      def map2[B, C](fb: F[B], f: (A, B) => C): F[C]
+      def map[B](f: A => B): F[B] =
+        fa.map2(unit, (a, _) => f(a))
+
+  trait Monad[F[_]] extends Applicative[F]:
+    extension[A](fa: F[A])
+      def flatMap[B](f: A => F[B]): F[B]
+      override def map[B](f: A => B): F[B] =
+        flatMap(a => pure(f(a)))
+      def map2[B, C](fb: F[B], f: (A, B) => C): F[C] =
+        flatMap(a => fb.map(b => f(a, b)))
+
+  given Monad[List] with
+    def pure[A](a: A) = List(a)
+    extension[A](fa: List[A])
+      def flatMap[B](f: A => List[B]) = fa.flatMap(f)
+
+  given Monad[Option] with
+    def pure[A](a: A) = Some(a)
+    extension[A](fa: Option[A])
+      def flatMap[B](f: A => Option[B]) = fa.flatMap(f)
+
+
+  opaque type Kleisli[F[_], A, B] = A => F[B]
+
+  extension [F[_], A, B](k: Kleisli[F, A, B])
+    def apply(a: A): F[B] = k(a)
+
+  object Kleisli:
+    def apply[F[_], A, B](f: A => F[B]): Kleisli[F, A, B] = f
+
+  given [F[_], A](using F: Monad[F]): Monad[[B] =>> Kleisli[F, A, B]] with
+    def pure[B](b: B) = Kleisli(_ => F.pure(b))
+    extension[B](k: Kleisli[F, A, B])
+      def flatMap[C](f: B => Kleisli[F, A, C]) =
+        a => k(a).flatMap(b => f(b)(a))
+
+end Structures
+
+@main def run =
+  import Structures.{*, given}
+  println(List(1, 2, 3).map2(List(4, 5, 6), (_, _)))
+
+  val p: Kleisli[Option, Int, Int] = Kleisli((x: Int) => if x % 2 == 0 then Some(x) else None)
+  val q: Kleisli[Option, Int, Int] = summon[Applicative[[B] =>> Kleisli[Option, Int, B]]].pure(20)
+  println(p.map2(q, _ + _)(42))
+