Follow singleton types when determining exclusivityInContext

Author: odersky

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

@@ -104,8 +104,14 @@ object Mutability:
         if tp.derivesFrom(defn.Caps_Stateful)
         then ctx.owner.inExclusivePartOf(tp.cls)
         else Exclusivity.OK
-      case tp @ TermRef(prefix: Capability, _) =>
-        prefix.exclusivityInContext(required).andAlso(tp.exclusivity(required))
+      case tp: TermRef =>
+        tp.info match
+          case tp1: SingletonType => tp1.exclusivityInContext(required)
+          case _ =>
+            val preEx = tp.prefix match
+              case prefix: Capability => prefix.exclusivityInContext(required)
+              case _ => Exclusivity.OK
+            preEx.andAlso(tp.exclusivity(required))
       case _ =>
         tp.exclusivity(required)
 

tests/neg-custom-args/captures/i25251.scala

@@ -0,0 +1,8 @@
+import caps.*
+trait A extends Stateful:
+  def f() = /* read-only f */
+    val ref: this.type = this
+    this.g()
+    ref.g()
+
+  update def g() = ???