BUG: Append pdf with named destination using numbers for pages

closes py-pdf#471

the issue was with named destination using numbers instead of indirect object to point pages. This is normally not expected.

pypdf/_writer.py

@@ -2755,8 +2755,25 @@ def merge(
         )  # need for the outline processing below
         for dest in reader._namedDests.values():
             arr = dest.dest_array
-            if isinstance(dest["/Page"], NullObject):
-                pass  # self.add_named_destination_array(dest["/Title"],arr)
+            if (  # noqa: SIM114
+                "/Names" in self._root_object
+                and dest["/Title"] in self._root_object["/Names"]["/Dests"]["/Names"]
+            ):
+                # already exists : should not duplicate it
+                pass
+            elif isinstance(dest["/Page"], NullObject):
+                pass
+            elif isinstance(dest["/Page"], int):
+                # the page reference is a page number normally not iaw Pdf Reference
+                # page numbers as int are normally accepted only in external goto
+                p = reader.pages[dest["/Page"]]
+                try:
+                    arr[NumberObject(0)] = NumberObject(
+                        srcpages[p.indirect_reference.idnum].page_number
+                    )
+                    self.add_named_destination_array(dest["/Title"], arr)
+                except KeyError:
+                    pass
             elif dest["/Page"].indirect_reference.idnum in srcpages:
                 arr[NumberObject(0)] = srcpages[
                     dest["/Page"].indirect_reference.idnum

tests/test_writer.py

@@ -1339,3 +1339,20 @@ def test_iss1767():
     name = "iss1723.pdf"
     in_pdf = PdfReader(BytesIO(get_pdf_from_url(url, name=name)))
     PdfWriter(clone_from=in_pdf)
+
+
+@pytest.mark.enable_socket()
+def test_named_dest_page_number():
+    """
+    Closes iss471
+    tests appending with named destinations as integers
+    """
+    url = "https://github.com/py-pdf/pypdf/files/10704333/central.pdf"
+    name = "central.pdf"
+    w = PdfWriter()
+    w.add_blank_page(100, 100)
+    w.append(BytesIO(get_pdf_from_url(url, name=name)), pages=[0, 1, 2])
+    assert len(w._root_object["/Names"]["/Dests"]["/Names"]) == 2
+    assert w._root_object["/Names"]["/Dests"]["/Names"][-1][0] == (1 + 1)
+    w.append(BytesIO(get_pdf_from_url(url, name=name)))
+    assert len(w._root_object["/Names"]["/Dests"]["/Names"]) == 6