Add test case for #18246

Author: Linyxus

tests/neg-custom-args/captures/cc-glb.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/cc-glb.scala:7:19 ----------------------------------------
+7 |  val x2: Foo[T] = x1  // error
+  |                   ^^
+  |                   Found:    (x1 : (Foo[T]^) & (Foo[Any]^{io}))
+  |                   Required: Foo[T]
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/cc-glb.scala

@@ -0,0 +1,8 @@
+import language.experimental.captureChecking
+trait Cap
+trait Foo[+T]
+
+def magic[T](io: Cap^, x: Foo[T]^{io}): Foo[T]^{} =
+  val x1: Foo[T]^{cap} & Foo[Any]^{io} = x
+  val x2: Foo[T] = x1  // error
+  x2  // boom, an impure value becomes pure