Fluidify explicit self types for non-CC classes

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

@@ -147,6 +147,17 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
     && !sym.owner.is(CaptureChecked)
     && !defn.isFunctionSymbol(sym.owner)
 
+  private def fluidifySelfInfo(info: Type)(using Context): Type = info match
+    case cinfo: ClassInfo =>
+      val selfInfo1 = cinfo.selfInfo match
+        case NoType => NoType
+        case tp: Type => decorate(tp, Function.const(CaptureSet.Fluid))
+        case self: Symbol => decorate(self.info, Function.const(CaptureSet.Fluid))
+      if selfInfo1 eq cinfo.selfInfo then cinfo
+      else cinfo.derivedClassInfo(selfInfo = selfInfo1)
+    case _ =>
+      info
+
   /** The symbol transformer of this phase.
    *   - Resets `private` flags of parameter accessors so that we can refine them
    *     in Setup if they have non-empty capture sets.
@@ -170,6 +181,10 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
             transformExplicitType(symd.info, sym)(using symCtx)
       if Synthetics.needsTransform(symd) then
         Synthetics.transform(symd, mappedInfo)
+      else if sym.isClass && !sym.is(CaptureChecked) then
+        val newInfo = fluidifySelfInfo(sym.info)
+        if newInfo ne sym.info then symd.copySymDenotation(info = newInfo)
+        else symd
       else if isPreCC(sym) then
         symd.copySymDenotation(info = fluidify(sym.info))
       else if symd.owner.isTerm

tests/pos/cc-self-fluid/FunSuite_1.scala

@@ -0,0 +1,3 @@
+trait FunSuite:
+  self: FunSuite =>
+  def run(): Unit

tests/pos/cc-self-fluid/Test_2.scala

@@ -0,0 +1,9 @@
+//> using options -language:experimental.captureChecking
+
+class IO extends caps.SharedCapability:
+  def println(msg: String): Unit = ()
+
+def test(io: IO): Unit =
+  class FunSuite1 extends FunSuite:
+    self: FunSuite1^ =>
+    def run(): Unit = io.println("Hello")

tests/pos/i25634/Stest_1.scala

@@ -0,0 +1,2 @@
+class A:
+  self: A =>

tests/pos/i25634/Stest_2.scala

@@ -0,0 +1,3 @@
+import language.experimental.captureChecking
+
+class B extends A, caps.SharedCapability