Add tests to document counter-intuitive version comparison

As discussed in #774, falling
back to lexicographic string comparison when using greater/less-than
operators can result in counter-intuitive behaviour on version-like
numeric strings.

Author: h4l

tests/test_markers.py

@@ -322,6 +322,15 @@ def test_environment_with_extra_none(self):
             ("extra == 'v8-dev'", {"extra": "v8-dev.0"}, True),
             # LHS and RHS are not valid versions, so equality is str comparison
             ("extra == 'v8-foo'", {"extra": "v8-foo.0"}, False),
+            #
+            # When using order comparisons, fallback to str when one or both are
+            # not valid versions can have counter-intuitive results.
+            #
+            # The gentoo value is not a valid version, lexicographic str
+            # comparison applies. This is True because '6' >= '2'.
+            ("platform_release >= '20'", {"platform_release": "6.7.0-gentoo"}, True),
+            # Whereas this is False because the gentoo value IS a valid version
+            ("platform_release >= '20'", {"platform_release": "6.7.0+gentoo"}, False),
         ],
     )
     def test_evaluates(self, marker_string, environment, expected):