Relax rule for stable in capture set

Author: noti0na1

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -788,7 +788,10 @@ object SymDenotations {
       * So the first call to a stable member might fail and/or produce side effects.
       */
     final def isStableMember(using Context): Boolean = {
-      def isUnstableValue = isOneOf(UnstableValueFlags) || isAllOf(InlineParam)
+      def isUnstableValue =
+        isOneOf(UnstableValueFlags)
+        || !ctx.mode.is(Mode.InCaptureSet) && info.isInstanceOf[ExprType]
+        || isAllOf(InlineParam)
       isType || is(StableRealizable) || exists && !isUnstableValue
     }
 

tests/neg-custom-args/captures/capt-wf-typer.scala

@@ -6,6 +6,6 @@ object foo
 
 def test(c: Cap, other: String): Unit =
   val x7: String^{c} = ??? // OK
-  val x8: String @retains[(x7 + x7)] = ??? // error
+  val x8: String @retains[""] = ??? // error
   val x9: String @retains[foo] = ??? // error
   ()