Allow candidates to not have an associated install requirement

src/pip/_internal/resolution/resolvelib/base.py

@@ -3,7 +3,7 @@ from pip._vendor.packaging.utils import canonicalize_name
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
-    from typing import Sequence, Set
+    from typing import Optional, Sequence, Set
 
     from pip._internal.req.req_install import InstallRequirement
     from pip._vendor.packaging.version import _BaseVersion
@@ -48,5 +48,5 @@ class Candidate(object):
         raise NotImplementedError("Override in subclass")
 
     def get_install_requirement(self):
-        # type: () -> InstallRequirement
+        # type: () -> Optional[InstallRequirement]
         raise NotImplementedError("Override in subclass")

src/pip/_internal/resolution/resolvelib/candidates.py

@@ -130,7 +130,7 @@ class LinkCandidate(Candidate):
         return [self._make_install_req(str(r)) for r in self.dist.requires()]
 
     def get_install_requirement(self):
-        # type: () -> InstallRequirement
+        # type: () -> Optional[InstallRequirement]
         return self._ireq
 
 
@@ -168,5 +168,8 @@ class ExtrasCandidate(LinkCandidate):
         return deps
 
     def get_install_requirement(self):
-        # type: () -> InstallRequirement
-        return self.base.get_install_requirement()
+        # type: () -> Optional[InstallRequirement]
+        # We don't return anything here, because we always
+        # depend on the base candidate, and we'll get the
+        # install requirement from that.
+        return None

src/pip/_internal/resolution/resolvelib/provider.py

@@ -37,7 +37,7 @@ class PipProvider(AbstractProvider):
         )
 
     def get_install_requirement(self, c):
-        # type: (Candidate) -> InstallRequirement
+        # type: (Candidate) -> Optional[InstallRequirement]
         return c.get_install_requirement()
 
     def identify(self, dependency):

src/pip/_internal/resolution/resolvelib/resolver.py

@@ -56,7 +56,8 @@ class Resolver(BaseResolver):
         req_set = RequirementSet(check_supported_wheels=check_supported_wheels)
         for candidate in self._result.mapping.values():
             ireq = provider.get_install_requirement(candidate)
-            req_set.add_named_requirement(ireq)
+            if ireq is not None:
+                req_set.add_named_requirement(ireq)
 
         return req_set