Tidy up handling of unexpected forms of constraint

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

@@ -2,6 +2,7 @@ import functools
 import logging
 
 from pip._vendor import six
+from pip._vendor.packaging.specifiers import SpecifierSet
 from pip._vendor.packaging.utils import canonicalize_name
 from pip._vendor.resolvelib import BaseReporter, ResolutionImpossible
 from pip._vendor.resolvelib import Resolver as RLResolver
@@ -17,7 +18,6 @@ from .factory import Factory
 if MYPY_CHECK_RUNNING:
     from typing import Dict, List, Optional, Set, Tuple
 
-    from pip._vendor.packaging.specifiers import SpecifierSet
     from pip._vendor.resolvelib.resolvers import Result
     from pip._vendor.resolvelib.structs import Graph
 
@@ -74,13 +74,29 @@ class Resolver(BaseResolver):
         requirements = []
         for req in root_reqs:
             if req.constraint:
-                assert req.name
-                assert req.specifier
+                # TODO: Add warnings to accompany these errors, explaining
+                # that these were undocumented behaviour of the old resolver
+                # and will be removed in the new resolver. We need to consider
+                # how we remember to remove these warnings when the new
+                # resolver becomes the default...
+                if not req.name:
+                    raise InstallationError(
+                        "Unnamed requirements are not allowed as constraints"
+                    )
+                if req.link:
+                    raise InstallationError(
+                        "Links are not allowed as constraints"
+                    )
+                if req.extras:
+                    raise InstallationError(
+                        "Constraints cannot have extras"
+                    )
+                specifier = req.specifier or SpecifierSet()
                 name = canonicalize_name(req.name)
                 if name in constraints:
-                    constraints[name] = constraints[name] & req.specifier
+                    constraints[name] = constraints[name] & specifier
                 else:
-                    constraints[name] = req.specifier
+                    constraints[name] = specifier
             else:
                 requirements.append(
                     self.factory.make_requirement_from_install_req(req)

tests/functional/test_new_resolver.py

@@ -548,6 +548,54 @@ def test_new_resolver_constraints(script):
     assert_not_installed(script, "constraint_only")
 
 
+def test_new_resolver_constraint_no_specifier(script):
+    "It's allowed (but useless...) for a constraint to have no specifier"
+    create_basic_wheel_for_package(script, "pkg", "1.0")
+    constraints_file = script.scratch_path / "constraints.txt"
+    constraints_file.write_text("pkg")
+    script.pip(
+        "install", "--unstable-feature=resolver",
+        "--no-cache-dir", "--no-index",
+        "--find-links", script.scratch_path,
+        "-c", constraints_file,
+        "pkg"
+    )
+    assert_installed(script, pkg="1.0")
+
+
+@pytest.mark.parametrize(
+    "constraint, error",
+    [
+        (
+            "dist.zip",
+            "Unnamed requirements are not allowed as constraints",
+        ),
+        (
+            "req @ https://example.com/dist.zip",
+            "Links are not allowed as constraints",
+        ),
+        (
+            "pkg[extra]",
+            "Constraints cannot have extras",
+        ),
+    ],
+)
+def test_new_resolver_constraint_reject_invalid(script, constraint, error):
+    create_basic_wheel_for_package(script, "pkg", "1.0")
+    constraints_file = script.scratch_path / "constraints.txt"
+    constraints_file.write_text(constraint)
+    result = script.pip(
+        "install", "--unstable-feature=resolver",
+        "--no-cache-dir", "--no-index",
+        "--find-links", script.scratch_path,
+        "-c", constraints_file,
+        "pkg",
+        expect_error=True,
+        expect_stderr=True,
+    )
+    assert error in result.stderr, str(result)
+
+
 def test_new_resolver_constraint_on_dependency(script):
     create_basic_wheel_for_package(script, "base", "1.0", depends=["dep"])
     create_basic_wheel_for_package(script, "dep", "1.0")