Do not check for trackable refs in BiTypeMap

Fixes scala#22437

The check crashes the compiler when mapping over a capture variable
in an upper bound of the form `CapSet^{C^}` as well as
path captures in function signatures. See the test
`capture-vars-subtyping2.scala` for examples.

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

@@ -6072,15 +6072,16 @@ object Types extends TypeUtils {
     /** A restriction of this map to a function on tracked CaptureRefs */
     def forward(ref: CaptureRef): CaptureRef =
       val result = this(ref)
-      def ensureTrackable(tp: Type): CaptureRef = tp match
+      /*def ensureTrackable(tp: Type): CaptureRef = tp match
         case tp: CaptureRef =>
           if tp.isTrackableRef then tp
           else ensureTrackable(tp.underlying)
         case tp: TypeAlias =>
           ensureTrackable(tp.alias)
         case _ =>
           assert(false, i"not a trackable captureRef ref: $result, ${result.underlyingIterator.toList}")
-      ensureTrackable(result)
+      ensureTrackable(result)*/
+      result.asInstanceOf[CaptureRef]
 
     /** A restriction of the inverse to a function on tracked CaptureRefs */
     def backward(ref: CaptureRef): CaptureRef = inverse(ref) match

tests/neg-custom-args/captures/capture-vars-subtyping2.scala

@@ -0,0 +1,44 @@
+import language.experimental.captureChecking
+import caps.*
+
+trait BoundsTest:
+
+  trait Bar { val f: () => Unit }
+  def bar(x: Bar^, y: () ->{x.f} Unit): Unit = ???
+
+  val b: Bar^ = ???
+
+  def testTransMixed[A^,
+                    B >: CapSet <: A,
+                    C >: CapSet <: CapSet^{B^},
+                    D >: CapSet <: C,
+                    E >: CapSet <: CapSet^{D^},
+                    F >: CapSet <: CapSet^{A^,b},
+                    X >: CapSet <: CapSet^{F^,D^},
+                    Y >: CapSet^{F^} <: CapSet^{F^,A^,b},
+                    Z >: CapSet^{b} <: CapSet^{b,Y^}] =
+    val e: E = ???
+    val e2: CapSet^{E^} = e
+    val ed: D = e
+    val ed2: CapSet^{D^} = e
+    val ec: C = e
+    val ec2: CapSet^{C^} = e
+    val eb: B = e
+    val eb2: CapSet^{B^} = e
+    val ea: A = e
+    val ea2: CapSet^{A^} = e
+    val ex: X = e            // error
+    val ex2: CapSet^{X^} = e // error
+    val f: F = ???
+    val f2: CapSet^{F^} = f
+    val y: Y = f
+    val y2: CapSet^{Y^} = f
+    val cb: CapSet^{b} = ???
+    val z: Z = cb
+    val z2: CapSet^{Z^} = cb
+
+  def callTransMixed =
+    val x, y, z: Bar^ = ???
+    testTransMixed[CapSet^{x,y,z}, CapSet^{x,y,z}, CapSet^{x,y,z}, CapSet^{x,y,z}, CapSet^{x,y,z}, CapSet^{x,y,z}, CapSet^{x,y,z}, CapSet^{x,y,z}, CapSet^{b,x,y,z}]
+    testTransMixed[CapSet^{x,y,z}, CapSet^{x,y}, CapSet^{x,y}, CapSet^{x}, CapSet^{}, CapSet^{b,x}, CapSet^{b}, CapSet^{b,x}, CapSet^{b}]
+    testTransMixed[CapSet^{x,y,z}, CapSet^{x,y}, CapSet^{x,y}, CapSet^{x}, CapSet^{}, CapSet^{b,x}, CapSet^{b}, CapSet^{b,x}, CapSet^{b,x,y,z}] // error