Keep deps so "dependsOn" is correct

Author: dwijnand

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

@@ -419,7 +419,9 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
           case prevEntry: TypeBounds =>
             adjustBounds(prevEntry, add = false)
           case _ =>
-        dropDeps(srcParam) // srcParam is instantiated, so its dependencies can be dropped
+        if entry != null && entry.exists then
+          // srcParam is instantiated, but keep dependencies to respond to "dependsOn"
+          adjustBounds(entry.bounds, add = true)
     this
   end adjustDeps
 

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

@@ -77,18 +77,9 @@ object ProtoTypes {
                   if !tvar.isInstantiated then
                     // Filter out any tvar that instantiating would further constrain the current constraint
                     // Similar to filterByDeps in interpolateTypeVars.
-                    // Also, filter out any tvar that is in the instantiation of another tvar
-                    // (that we're not also trying to instantiate)
-                    // For example, in tests/pos/i21981.scala
-                    // when testing the compatibility of `.map2[?K]` on receiver `map0[?B]`
-                    // the tvars for B and K unified, instantiating `B := K`,
-                    // so we can't instantiate away `K` as it would incorrectly define `B`.
                     val excluded = ctx.typerState.ownedVars.filter(!_.isInstantiated)
-                    var isInst = false
-                    ctx.typerState.constraint.foreachTypeVar: tvar1 =>
-                      isInst ||= !excluded.contains(tvar1) && tvar1.instanceOpt == tvar
-                    val aboveOK = !isInst && !ctx.typerState.constraint.dependsOn(tvar, excluded, co = true)
-                    val belowOK = !isInst && !ctx.typerState.constraint.dependsOn(tvar, excluded, co = false)
+                    val aboveOK = !ctx.typerState.constraint.dependsOn(tvar, excluded, co = true)
+                    val belowOK = !ctx.typerState.constraint.dependsOn(tvar, excluded, co = false)
                     if aboveOK then
                       tvar.instantiate(fromBelow = false)
                     else if belowOK then

tests/pos/i21981.contrak.scala

@@ -0,0 +1,27 @@
+case class Inv[T](x: T)
+class Contra[-ContraParam](x: ContraParam)
+
+trait Ops[F[_], A]:
+  def map0[B](f0: A => Contra[B]): F[B] = ???
+
+trait Functor1[G[_]]
+
+trait Functor2[H[_]]
+
+trait Ref[I[_], +E]
+
+class Test:
+  given [J[_]](using J: Functor1[J]): Functor2[J] with
+    extension [K](jk: J[Contra[K]])
+      def map2[L](f2: K => L): J[L] = ???
+
+  def t1[
+    M[_[t]],
+    N[_],
+  ](using N: Functor1[N]): Unit =
+
+    val x3: Ops[N, M[[t] =>> Ref[N, t]]] = ???
+
+    val x2: N[(M[N], M[[t] =>> Ref[N, t]])] = x3
+      .map0 { refs             => Contra[Contra[(Nothing, M[[t] =>> Ref[N, t]])]](???) }
+      .map2 { case (not, refs) => (???, refs) }