Strengthen lambda cleanup restrictions

Author: dwijnand

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

@@ -77,13 +77,22 @@ 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 the instantiation 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)
-                    val aboveOK = !ctx.typerState.constraint.dependsOn(tvar, excluded, co = true)
-                    val belowOK = !ctx.typerState.constraint.dependsOn(tvar, excluded, co = false)
-                    if belowOK then
-                      tvar.instantiate(fromBelow = true)
-                    else if aboveOK then
+                    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)
+                    if aboveOK then
                       tvar.instantiate(fromBelow = false)
+                    else if belowOK then
+                      tvar.instantiate(fromBelow = true)
 
               // commit any remaining changes in typer state
               newctx.typerState.commit()