Test case for object captures (#24670)

Closes #24667

Author: odersky

tests/neg-custom-args/captures/i24667.check

@@ -0,0 +1,18 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i24667.scala:10:18 ---------------------------------------
+10 |  val _: AnyRef = w // error on this line
+   |                  ^
+   |                  Found:    (w : O.type)
+   |                  Required: AnyRef
+   |
+   |                  Note that capability c is not included in capture set {}.
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i24667.scala:11:22 ---------------------------------------
+11 |  val _: AnyRef^{c} = w // error on this line, since d is also captured
+   |                      ^
+   |                      Found:    (w : O.type)
+   |                      Required: Object^{c}
+   |
+   |                      Note that capability O is not included in capture set {c}.
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/i24667.scala

@@ -0,0 +1,13 @@
+import language.experimental.captureChecking
+
+def foo(c: AnyRef^, d: AnyRef^, e: AnyRef^): Unit =
+  trait I
+  object O:
+    val x = c
+    def foo = println(d)
+
+  val w: O.type = O  // O is not pure
+  val _: AnyRef = w // error on this line
+  val _: AnyRef^{c} = w // error on this line, since d is also captured
+  val _: AnyRef^{c, d} = w // ok
+