Properly yield results from html5lib parsing

The earlier variant _returned_ an iterable object from a generator. This
did not properly handle the fallback, resulting in the html5lib code
path not being executed.

news/10846.bugfix.rst

@@ -0,0 +1 @@
+Restore equivalence with earlier version of pip, in `--use-deprecated=html5lib`.

src/pip/_internal/index/collector.py

@@ -343,7 +343,8 @@ def parse_links(page: "HTMLPage", use_deprecated_html5lib: bool) -> Iterable[Lin
     Parse an HTML document, and yield its anchor elements as Link objects.
     """
     if use_deprecated_html5lib:
-        return _parse_links_html5lib(page)
+        yield from _parse_links_html5lib(page)
+        return
 
     parser = HTMLLinkParser()
     encoding = page.encoding or "utf-8"

tests/unit/test_collector.py

@@ -539,6 +539,18 @@ def test_parse_links_caches_same_page_by_url() -> None:
     assert "pkg2" in parsed_links_3[0].url
 
 
+def test_parse_link_handles_deprecated_usage_properly() -> None:
+    html = b'<a href="/pkg1-1.0.tar.gz"><a href="/pkg1-2.0.tar.gz">'
+    url = "https://example.com/simple/"
+    page = HTMLPage(html, encoding=None, url=url)
+
+    parsed_links = list(parse_links(page, use_deprecated_html5lib=True))
+
+    assert len(parsed_links) == 2
+    assert "pkg1-1.0" in parsed_links[0].url
+    assert "pkg1-2.0" in parsed_links[1].url
+
+
 @mock.patch("pip._internal.index.collector.raise_for_status")
 def test_request_http_error(
     mock_raise_for_status: mock.Mock, caplog: pytest.LogCaptureFixture