Fix default self types of capabilities

They still were read-only, but all Capabilities implicitly capture `cap` now.

Author: odersky

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -689,9 +689,9 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
                 // neither pure, nor are publicily extensible with an unconstrained self type.
                 val cs = CaptureSet.ProperVar(cls, CaptureSet.emptyRefs, nestedOK = false, isRefining = false)
                 if cls.derivesFrom(defn.Caps_Capability) then
-                  // If cls is a capability class, we need to add a fresh readonly capability to
-                  // ensure we cannot treat the class as pure.
-                  CaptureSet.fresh(cls, cls.thisType, Origin.InDecl(cls)).readOnly.subCaptures(cs)
+                  // If cls is a capability class, we need to add a fresh capability to ensure
+                  // we cannot treat the class as pure.
+                  CaptureSet.fresh(cls, cls.thisType, Origin.InDecl(cls)).subCaptures(cs)
                 CapturingType(cinfo.selfType, cs)
 
             // Compute new parent types

tests/neg-custom-args/captures/linear-buffer.check

@@ -3,8 +3,16 @@
   |                           ^^^^^^^^^^^^^
   |      Separation failure: method append's result type BadBuffer[T]^ hides non-local this of class class BadBuffer.
   |      The access must be in a consume method to allow this.
+-- Error: tests/neg-custom-args/captures/linear-buffer.scala:7:29 ------------------------------------------------------
+7 |    def bar: BadBuffer[T]^ = this // error // error
+  |                             ^^^^
+  |           Separation failure: Illegal access to {BadBuffer.this} which is hidden by the previous definition
+  |           of method append with result type BadBuffer[T]^.
+  |           This type hides capabilities  {BadBuffer.this}
+  |
+  |           where:    ^ refers to a fresh root capability classified as Mutable in the result type of method append
 -- Error: tests/neg-custom-args/captures/linear-buffer.scala:7:13 ------------------------------------------------------
-7 |    def bar: BadBuffer[T]^ = this // error
+7 |    def bar: BadBuffer[T]^ = this // error // error
   |             ^^^^^^^^^^^^^
   |         Separation failure: method bar's result type BadBuffer[T]^ hides non-local this of class class BadBuffer.
   |         The access must be in a consume method to allow this.

tests/neg-custom-args/captures/linear-buffer.scala

@@ -4,7 +4,7 @@ import language.experimental.captureChecking
 class BadBuffer[T] extends Mutable:
   update def append(x: T): BadBuffer[T]^ = this // error
   def foo =
-    def bar: BadBuffer[T]^ = this // error
+    def bar: BadBuffer[T]^ = this // error // error
     bar
 
 class Buffer[T] extends Mutable: