Tweak tparam unification to work with lambda cleanup

Author: dwijnand

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

@@ -436,8 +436,17 @@ trait ConstraintHandling {
 
     val level1 = nestingLevel(p1)
     val level2 = nestingLevel(p2)
-    val pKept    = if level1 <= level2 then p1 else p2
-    val pRemoved = if level1 <= level2 then p2 else p1
+    val p1Wins = if level1 == level2 then
+      // If the nesting levels match, then we would prefer to unify to the outer most parameter.
+      // For instance in pos/i21981, while running `normalizedCompatible` against `.map2`,
+      // we want to unify to B over K, to allow easily removing K by just instantiating it.
+      def preferP1(ctx: Context): Boolean =
+        val c = ctx.typerState.constraint
+        !c.contains(p2) || c.contains(p1) && preferP1(ctx.outer)
+      preferP1(ctx)
+    else level1 <= level2
+    val pKept    = if p1Wins then p1 else p2
+    val pRemoved = if p1Wins then p2 else p1
 
     val down = constraint.exclusiveLower(p2, p1)
     val up = constraint.exclusiveUpper(p1, p2)

tests/pos/i21981.scala

@@ -0,0 +1,28 @@
+trait Ops[F[_], A]:
+  def map0[B](f0: A => B): F[B] = ???
+
+trait Functor1[G[_]]
+
+trait Functor2[H[_]]:
+  extension [C](hc: H[C])
+    def map2[D](f1: C => D): H[D]
+
+trait Ref[I[_], +E]
+
+final class Cov[+F]
+
+class Test:
+  given [J[_]](using J: Functor1[J]): Functor2[J] with
+    extension [K](jk: J[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             => (???, refs) }
+      .map2 { case (not, refs) => (???, refs) }