Duplicate distributions with distinct extras

Any duplicate distributions will currently raise an error, even if
their extras are different: for example, 'pip install bar bar[foo]'.
Detect this situation, and union the extras together.

Closes #3189

pip/req/req_set.py

@@ -238,6 +238,15 @@ def add_requirement(self, install_req, parent_req_name=None):
                 existing_req = None
             if (parent_req_name is None and existing_req and not
                     existing_req.constraint):
+                if existing_req.name == install_req.name:
+                    # If the extras do not match, then union them together to
+                    # support install bar bar[foo]. If they do match, the
+                    # InstallationError will be raised as per normal.
+                    if existing_req.extras != install_req.extras:
+                        existing_req.extras = tuple(
+                            set(existing_req.extras).union(
+                                set(install_req.extras)))
+                        return [existing_req]
                 raise InstallationError(
                     'Double requirement given: %s (already in %s, name=%r)'
                     % (install_req, existing_req, name))

tests/data/packages/LocalExtras/setup.py

@@ -24,6 +24,6 @@ def path_to_url(path):
     name='LocalExtras',
     version='0.0.1',
     packages=find_packages(),
-    extras_require={ 'bar': ['simple'] },
+    extras_require={ 'bar': ['simple'], 'baz': ['singlemodule'] },
     dependency_links=[DEP_URL]
 )

tests/functional/test_install_reqs.py

@@ -313,3 +313,20 @@ def test_constrained_to_url_install_same_url(script, data):
         'install', '--no-index', '-f', data.find_links, '-c',
         script.scratch_path / 'constraints.txt', to_install)
     assert 'Running setup.py install for singlemodule' in result.stdout
+
+
+def test_install_distribution_full_union(script, data):
+    to_install = data.packages.join("LocalExtras")
+    result = script.pip_install_local(
+        to_install, to_install + "[bar]", to_install + "[baz]")
+    assert 'Running setup.py install for LocalExtras' in result.stdout
+    assert script.site_packages / 'simple' in result.files_created
+    assert script.site_packages / 'singlemodule.py' in result.files_created
+
+
+def test_install_distribution_duplicate_extras(script, data):
+    to_install = data.packages.join("LocalExtras")
+    package_name = to_install + "[bar]"
+    with pytest.raises(AssertionError):
+        result = script.pip_install_local(package_name, package_name)
+        assert 'Double requirement given: %s' % package_name in result.stderr