Fluidify explicit self types for non-CC classes

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

@@ -170,6 +170,10 @@ 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 = fluidify(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
@@ -949,6 +953,13 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
    */
   private def fluidify(using Context) = new TypeMap:
     def apply(t: Type): Type = t match
+      case cinfo: ClassInfo =>
+        val selfInfo1: TypeOrSymbol = cinfo.selfInfo match
+          case selfInfo: Type =>
+            inContext(ctx.withOwner(cinfo.cls))(atVariance(0)(this(selfInfo)))
+          case self: Symbol =>
+            inContext(ctx.withOwner(cinfo.cls))(atVariance(0)(this(self.info)))
+        cinfo.derivedClassInfo(selfInfo = selfInfo1)
       case t: MethodType =>
         mapOver(t)
       case t: TypeLambda =>

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