Add test cases for #22103

Author: Linyxus

tests/pos-custom-args/captures/i22103-alt.scala

@@ -0,0 +1,8 @@
+import language.experimental.captureChecking
+import caps.*
+
+trait F[+Cap^]
+def expect[C^](x: F[C]): x.type = x
+
+def test[C^](x: F[C], io: () => Unit): Unit =
+  val t1 = expect[CapSet^{C^}](x)

tests/pos-custom-args/captures/i22103.scala

@@ -0,0 +1,15 @@
+import language.experimental.captureChecking
+import caps.*
+
+abstract class Source[+T, Cap^]:
+  def transformValuesWith[U](f: (T -> U)^{Cap^}): Source[U, Cap]^{this, f} = ???
+
+def race[T, Cap^](sources: Source[T, Cap]^{Cap^}*): Source[T, Cap]^{Cap^} = ???
+
+def either[T1, T2, Cap^](
+    //io: () => Unit,
+    src1: Source[T1, Cap]^{Cap^},
+    src2: Source[T2, Cap]^{Cap^}): Source[Either[T1, T2], Cap]^{Cap^} =
+  val left = src1.transformValuesWith(Left(_))
+  val right = src2.transformValuesWith(Right(_))
+  race[Either[T1, T2], CapSet^{Cap^}](left, right) // <- rejected