Add sealed refs test and fix check files of other tests

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

@@ -12,4 +12,4 @@
 -- Error: tests/neg-custom-args/captures/cc-this.scala:17:8 ------------------------------------------------------------
 17 |  class C4(val f: () => Int) extends C3 // error
    |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |  reference (C4.this.f : () => Int) is not included in the allowed capture set {} of pure base class class C3
+   |reference (C4.this.f : () => Int) captured by this self type is not included in the allowed capture set {} of pure base class class C3

tests/neg-custom-args/captures/cc-this2.check

@@ -1,6 +1,12 @@
 
--- Error: tests/neg-custom-args/captures/cc-this2/D_2.scala:2:6 --------------------------------------------------------
+-- Error: tests/neg-custom-args/captures/cc-this2/D_2.scala:3:8 --------------------------------------------------------
+3 |  this: D^ => // error
+  |        ^^
+  |reference (caps.cap : caps.Cap) captured by this self type is not included in the allowed capture set {} of pure base class class C
+-- [E058] Type Mismatch Error: tests/neg-custom-args/captures/cc-this2/D_2.scala:2:6 -----------------------------------
 2 |class D extends C: // error
-  |^
-  |reference (caps.cap : caps.Cap) is not included in the allowed capture set {} of pure base class class C
-3 |  this: D^ =>
+  |      ^
+  |      illegal inheritance: self type D^ of class D does not conform to self type C
+  |      of parent class C
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/cc-this2/D_2.scala

@@ -1,3 +1,3 @@
 
 class D extends C: // error
-  this: D^ =>
+  this: D^ => // error

tests/neg-custom-args/captures/exception-definitions.check

@@ -1,8 +1,7 @@
--- Error: tests/neg-custom-args/captures/exception-definitions.scala:2:6 -----------------------------------------------
-2 |class Err extends Exception: // error
-  |^
-  |reference (caps.cap : caps.Cap) is not included in the allowed capture set {} of pure base class class Throwable
-3 |  self: Err^ =>
+-- Error: tests/neg-custom-args/captures/exception-definitions.scala:3:8 -----------------------------------------------
+3 |  self: Err^ => // error
+  |        ^^^^
+  |reference (caps.cap : caps.Cap) captured by this self type is not included in the allowed capture set {} of pure base class class Throwable
 -- Error: tests/neg-custom-args/captures/exception-definitions.scala:7:12 ----------------------------------------------
 7 |    val x = c  // error
   |            ^

tests/neg-custom-args/captures/exception-definitions.scala

@@ -1,6 +1,6 @@
 
-class Err extends Exception: // error
-  self: Err^ =>
+class Err extends Exception:
+  self: Err^ => // error
 
 def test(c: Any^) =
   class Err2 extends Exception:

tests/neg-custom-args/captures/sealed-refs.scala

@@ -0,0 +1,42 @@
+class Ref[sealed A](init: A):
+  this: Ref[A]^ =>
+  private var x: A = init
+  def get: A = x
+  def set(x: A): Unit = this.x = x
+
+class It[X]:
+  this: It[X]^ =>
+
+def f1[B1](x: B1, next: B1 -> B1) =
+  var r = x    // ok
+  r = next(x)
+  r
+
+def f2[B2](x: B2, next: B2 -> B2) =
+  val r = Ref[B2](x)    // error
+  r.set(next(x))
+  r.get
+
+def g[sealed B](x: B, next: B -> B) =
+  val r = Ref[B](x)     // ok
+  r.set(next(x))
+  r.get
+
+import annotation.unchecked.uncheckedCaptures
+
+def h[B](x: B, next: B -> B) =
+  val r = Ref[B @uncheckedCaptures](x)  // ok
+  r.set(next(x))
+  r.get
+
+def f3[B](x: B, next: B -> B) =
+  val r: Ref[B^{cap[f3]}] = Ref[B^{cap[f3]}](x)    // error
+  r.set(next(x))
+  val y = r.get
+  ()
+
+def f4[B](x: B, next: B -> B) =
+  val r: Ref[B]^{cap[f4]} = Ref[B](x)    // error
+  r.set(next(x))
+  val y = r.get
+  ()