fix possible error when finalizing `install_requires`

setuptools/dist.py

@@ -369,11 +369,12 @@ class Distribution(Distribution_parse_config_files, _Distribution):
         Move requirements in `install_requires` that
         are using environment markers to `extras_require`.
         """
-        if not self.install_requires:
+        if not getattr(self, 'install_requires', None):
             return
+        extras_require = getattr(self, 'extras_require', None)
         extras_require = defaultdict(list, (
             (k, list(pkg_resources.parse_requirements(v)))
-            for k, v in (self.extras_require or {}).items()
+            for k, v in (extras_require or {}).items()
         ))
         install_requires = []
         for r in pkg_resources.parse_requirements(self.install_requires):

setuptools/tests/test_virtualenv.py

+import glob
 import os
 
 from pytest import yield_fixture
@@ -29,3 +30,21 @@ def test_clean_env_install(bare_virtualenv):
         'cd {source}',
         'python setup.py install',
     )).format(source=SOURCE_DIR))
+
+def test_pip_upgrade_from_source(virtualenv):
+    """
+    Check pip can upgrade setuptools from source.
+    """
+    dist_dir = virtualenv.workspace
+    # Generate source distribution / wheel.
+    virtualenv.run(' && '.join((
+        'cd {source}',
+        'python setup.py -q sdist -d {dist}',
+        'python setup.py -q bdist_wheel -d {dist}',
+    )).format(source=SOURCE_DIR, dist=dist_dir))
+    sdist = glob.glob(os.path.join(dist_dir, '*.zip'))[0]
+    wheel = glob.glob(os.path.join(dist_dir, '*.whl'))[0]
+    # Then update from wheel.
+    virtualenv.run('pip install ' + wheel)
+    # And finally try to upgrade from source.
+    virtualenv.run('pip install --no-cache-dir --upgrade ' + sdist)