Fix #5521: prefix not checked in realizability check

We were forgetting to check the prefix in the uncached case.

Author: abeln

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

@@ -77,8 +77,11 @@ class CheckRealizable(implicit ctx: Context) {
           else if (!isLateInitialized(sym)) realizability(tp.prefix)
           else if (!sym.isEffectivelyFinal) new NotFinal(sym)
           else realizability(tp.info).mapError(r => new ProblemInUnderlying(tp.info, r))
-        if (r == Realizable) sym.setFlag(Stable)
-        r
+        val r1 = if (r == Realizable) {
+          sym.setFlag(Stable)
+          realizability(tp.prefix)
+        } else r
+        r1
       }
     case _: SingletonType | NoPrefix =>
       Realizable

tests/neg/i5521.scala

@@ -0,0 +1,12 @@
+class Hello {
+  class Foo {
+    class Bar
+    final lazy val s: Bar = ???
+  }
+
+  lazy val foo: Foo = ???
+
+  val x: foo.s.type = ???   // error: `foo` must be final since it's a lazy val
+  val x2: foo.s.type = ???  // error: `foo` must be final since it's a lazy val
+}
+