Refine isEffectivelyFinal to avoid no-owner crash

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

@@ -1220,7 +1220,7 @@ object SymDenotations {
       || is(Inline, butNot = Deferred)
       || is(JavaDefinedVal, butNot = Method)
       || isConstructor
-      || !owner.isExtensibleClass && !is(Deferred)
+      || exists && !owner.isExtensibleClass && !is(Deferred)
       	// Deferred symbols can arise through parent refinements under x.modularity.
       	// For them, the overriding relationship reverses anyway, so
       	// being in a final class does not mean the symbol cannot be

tests/neg/i23637.check

@@ -0,0 +1,6 @@
+-- [E083] Type Error: tests/neg/i23637.scala:6:9 -----------------------------------------------------------------------
+6 |  export foo.pin.* // error: (because we need reflection to get at foo.pin)
+  |         ^^^^^^^
+  |         (Test.foo.pin : Object) is not a valid export prefix, since it is not an immutable path
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i23637.scala

@@ -0,0 +1,12 @@
+trait Foo extends reflect.Selectable
+object Test:
+  val foo = new Foo:
+    object pin:
+      val x = 1
+  export foo.pin.* // error: (because we need reflection to get at foo.pin)
+
+object OK:
+  object Foo:
+    object pin:
+      val x = 1
+  export Foo.pin.*