Fix dropped markers in dependency walk

This is a backport of #4686. Note that the test is changed due to a
regression in `master` -- somewhere, the ability to drop two markers
that negate each other was lost.

Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>

poetry/packages/locker.py

@@ -245,16 +245,15 @@ def __walk_dependency_level(
                         locked_package.to_dependency().constraint
                     )
 
-                if key not in nested_dependencies:
-                    for require in locked_package.requires:
-                        if require.marker.is_empty():
-                            require.marker = requirement.marker
-                        else:
-                            require.marker = require.marker.intersect(
-                                requirement.marker
-                            )
+                for require in locked_package.requires:
+                    if require.marker.is_empty():
+                        require.marker = requirement.marker
+                    else:
+                        require.marker = require.marker.intersect(requirement.marker)
+
+                    require.marker = require.marker.intersect(locked_package.marker)
 
-                        require.marker = require.marker.intersect(locked_package.marker)
+                    if key not in nested_dependencies:
                         next_level_dependencies.append(require)
 
             if requirement.name in project_level_dependencies and level == 0:

tests/utils/test_exporter.py

@@ -752,6 +752,79 @@ def test_exporter_can_export_requirements_txt_with_nested_packages_cyclic(
     assert expected == content
 
 
+def test_exporter_can_export_requirements_txt_with_nested_packages_and_multiple_markers(
+    tmp_dir, poetry
+):
+    poetry.locker.mock_lock_data(
+        {
+            "package": [
+                {
+                    "name": "foo",
+                    "version": "1.2.3",
+                    "category": "main",
+                    "optional": False,
+                    "python-versions": "*",
+                    "dependencies": {
+                        "bar": [
+                            {
+                                "version": ">=1.2.3,<7.8.10",
+                                "markers": 'platform_system != "Windows"',
+                            },
+                            {
+                                "version": ">=4.5.6,<7.8.10",
+                                "markers": 'platform_system == "Windows"',
+                            },
+                        ]
+                    },
+                },
+                {
+                    "name": "bar",
+                    "version": "7.8.9",
+                    "category": "main",
+                    "optional": True,
+                    "python-versions": "*",
+                    "dependencies": {
+                        "baz": {
+                            "version": "!=10.11.12",
+                            "markers": 'platform_system == "Windows"',
+                        }
+                    },
+                },
+                {
+                    "name": "baz",
+                    "version": "10.11.13",
+                    "category": "main",
+                    "optional": True,
+                    "python-versions": "*",
+                },
+            ],
+            "metadata": {
+                "python-versions": "*",
+                "content-hash": "123456789",
+                "hashes": {"foo": [], "bar": [], "baz": []},
+            },
+        }
+    )
+    set_package_requires(poetry)
+
+    exporter = Exporter(poetry)
+
+    exporter.export(
+        "requirements.txt", Path(tmp_dir), "requirements.txt", with_hashes=False
+    )
+
+    with (Path(tmp_dir) / "requirements.txt").open(encoding="utf-8") as f:
+        content = f.read()
+
+    expected = """\
+bar==7.8.9
+baz==10.11.13; platform_system == "Windows"
+foo==1.2.3
+"""
+
+    assert expected == content
+
+
 def test_exporter_can_export_requirements_txt_with_git_packages_and_markers(
     tmp_dir, poetry
 ):