Fixes #22437

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

Author: bracevac

Diff for: compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -570,7 +570,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
               // For example, `(x: T, y: x.f.type) => Unit`. In this case, when we
               // substitute `x.f.type`, `x` becomes a `TermParamRef`. But the new method
               // type is still under initialization and `paramInfos` is still `null`,
-              // so the new `NamedType` will not have a denoation.
+              // so the new `NamedType` will not have a denotation.
               def adaptedInfo(psym: Symbol, info: mt.PInfo): mt.PInfo = mt.companion match
                 case mtc: MethodTypeCompanion => mtc.adaptParamInfo(psym, info).asInstanceOf[mt.PInfo]
                 case _ => info

Diff for: compiler/src/dotty/tools/dotc/core/Types.scala

@@ -6073,6 +6073,9 @@ object Types extends TypeUtils {
     def forward(ref: CaptureRef): CaptureRef =
       val result = this(ref)
       def ensureTrackable(tp: Type): CaptureRef = tp match
+        /* Issue #22437: handle case when info is not yet available during postProcess in CC setup */
+        case tp: (TypeParamRef | TermRef) if tp.underlying == NoType =>
+          tp
         case tp: CaptureRef =>
           if tp.isTrackableRef then tp
           else ensureTrackable(tp.underlying)
@@ -6084,6 +6087,9 @@ object Types extends TypeUtils {
 
     /** A restriction of the inverse to a function on tracked CaptureRefs */
     def backward(ref: CaptureRef): CaptureRef = inverse(ref) match
+      /* Ensure bijection for issue #22437 fix in method forward above:  */
+      case result: (TypeParamRef | TermRef) if result.underlying == NoType =>
+        result
       case result: CaptureRef if result.isTrackableRef => result
   end BiTypeMap
 

Diff for: 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