Fix cases of package[...]==version

Author: astrofrog

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

@@ -3,7 +3,7 @@
 from pip._vendor.packaging.specifiers import SpecifierSet
 from pip._vendor.packaging.utils import NormalizedName, canonicalize_name
 
-from pip._internal.req.constructors import install_req_drop_extras
+from pip._internal.req.constructors import install_req_drop_extras, EXPLICIT_EMPTY_EXTRAS
 from pip._internal.req.req_install import InstallRequirement
 
 from .base import Candidate, CandidateLookup, Requirement, format_name
@@ -128,6 +128,8 @@ class SpecifierWithoutExtrasRequirement(SpecifierRequirement):
     def __init__(self, ireq: InstallRequirement) -> None:
         assert ireq.link is None, "This is a link, not a specifier"
         self._ireq = install_req_drop_extras(ireq)
+        if ireq.extras:
+            self._ireq.extras = {EXPLICIT_EMPTY_EXTRAS}
         self._equal_cache: Optional[str] = None
         self._hash: Optional[int] = None
         self._extras = frozenset(canonicalize_name(e) for e in self._ireq.extras)