Warn on unsupported use of constraints

src/pip/_internal/req/req_install.py

@@ -864,3 +864,35 @@ def install(
             raise
 
         self.install_succeeded = success
+
+
+def check_invalid_constraint_type(req):
+    # type: (InstallRequirement) -> str
+
+    # Check for unsupported forms
+    problem = ""
+    if not req.name:
+        problem = "Unnamed requirements are not allowed as constraints"
+    elif req.link:
+        problem = "Links are not allowed as constraints"
+    elif req.extras:
+        problem = "Constraints cannot have extras"
+
+    if problem:
+        deprecated(
+            reason=(
+                "Constraints are only allowed to take the form of a package "
+                "name and a version specifier. Other forms were originally "
+                "permitted as an accident of the implementation, but were "
+                "undocumented. The new implementation of the resolver no "
+                "longer supports these forms."
+            ),
+            replacement=(
+                "replacing the constraint with a requirement."
+            ),
+            # No plan yet for when the new resolver becomes default
+            gone_in=None,
+            issue=8210
+        )
+
+    return problem

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

@@ -28,6 +28,7 @@
     HashErrors,
     UnsupportedPythonVersion,
 )
+from pip._internal.req.req_install import check_invalid_constraint_type
 from pip._internal.req.req_set import RequirementSet
 from pip._internal.resolution.base import BaseResolver
 from pip._internal.utils.compatibility_tags import get_supported
@@ -167,6 +168,8 @@ def resolve(self, root_reqs, check_supported_wheels):
             check_supported_wheels=check_supported_wheels
         )
         for req in root_reqs:
+            if req.constraint:
+                check_invalid_constraint_type(req)
             requirement_set.add_requirement(req)
 
         # Actually prepare the files, and collect any exceptions. Most hash

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

@@ -7,10 +7,10 @@
 from pip._vendor.resolvelib import Resolver as RLResolver
 
 from pip._internal.exceptions import InstallationError
+from pip._internal.req.req_install import check_invalid_constraint_type
 from pip._internal.req.req_set import RequirementSet
 from pip._internal.resolution.base import BaseResolver
 from pip._internal.resolution.resolvelib.provider import PipProvider
-from pip._internal.utils.deprecation import deprecated
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 from .factory import Factory
@@ -32,37 +32,6 @@
 logger = logging.getLogger(__name__)
 
 
-def reject_invalid_constraint_types(req):
-    # type: (InstallRequirement) -> None
-
-    # Check for unsupported forms
-    problem = ""
-    if not req.name:
-        problem = "Unnamed requirements are not allowed as constraints"
-    elif req.link:
-        problem = "Links are not allowed as constraints"
-    elif req.extras:
-        problem = "Constraints cannot have extras"
-
-    if problem:
-        deprecated(
-            reason=(
-                "Constraints are only allowed to take the form of a package "
-                "name and a version specifier. Other forms were originally "
-                "permitted as an accident of the implementation, but were "
-                "undocumented. The new implementation of the resolver no "
-                "longer supports these forms."
-            ),
-            replacement=(
-                "replacing the constraint with a requirement."
-            ),
-            # No plan yet for when the new resolver becomes default
-            gone_in=None,
-            issue=8210
-        )
-        raise InstallationError(problem)
-
-
 class Resolver(BaseResolver):
     _allowed_strategies = {"eager", "only-if-needed", "to-satisfy-only"}
 
@@ -108,7 +77,9 @@ def resolve(self, root_reqs, check_supported_wheels):
         for req in root_reqs:
             if req.constraint:
                 # Ensure we only accept valid constraints
-                reject_invalid_constraint_types(req)
+                problem = check_invalid_constraint_type(req)
+                if problem:
+                    raise InstallationError(problem)
 
                 name = canonicalize_name(req.name)
                 if name in constraints:

tests/functional/test_install_reqs.py

@@ -344,6 +344,7 @@ def test_constraints_constrain_to_local_editable(
     result = script.pip(
         'install', '--no-index', '-f', data.find_links, '-c',
         script.scratch_path / 'constraints.txt', 'singlemodule',
+        allow_stderr_warning=True,
         expect_error=use_new_resolver
     )
     if use_new_resolver:
@@ -360,6 +361,7 @@ def test_constraints_constrain_to_local(script, data, use_new_resolver):
     result = script.pip(
         'install', '--no-index', '-f', data.find_links, '-c',
         script.scratch_path / 'constraints.txt', 'singlemodule',
+        allow_stderr_warning=True,
         expect_error=use_new_resolver
     )
     if use_new_resolver:
@@ -375,6 +377,7 @@ def test_constrained_to_url_install_same_url(script, data, use_new_resolver):
     result = script.pip(
         'install', '--no-index', '-f', data.find_links, '-c',
         script.scratch_path / 'constraints.txt', to_install,
+        allow_stderr_warning=True,
         expect_error=use_new_resolver
     )
     if use_new_resolver:
@@ -425,6 +428,7 @@ def test_install_with_extras_from_constraints(script, data, use_new_resolver):
     )
     result = script.pip_install_local(
         '-c', script.scratch_path / 'constraints.txt', 'LocalExtras',
+        allow_stderr_warning=True,
         expect_error=use_new_resolver
     )
     if use_new_resolver:
@@ -456,6 +460,7 @@ def test_install_with_extras_joined(script, data, use_new_resolver):
     )
     result = script.pip_install_local(
         '-c', script.scratch_path / 'constraints.txt', 'LocalExtras[baz]',
+        allow_stderr_warning=True,
         expect_error=use_new_resolver
     )
     if use_new_resolver:
@@ -472,6 +477,7 @@ def test_install_with_extras_editable_joined(script, data, use_new_resolver):
     )
     result = script.pip_install_local(
         '-c', script.scratch_path / 'constraints.txt', 'LocalExtras[baz]',
+        allow_stderr_warning=True,
         expect_error=use_new_resolver
     )
     if use_new_resolver:
@@ -510,6 +516,7 @@ def test_install_distribution_union_with_constraints(
         "{to_install}[bar]".format(**locals()))
     result = script.pip_install_local(
         '-c', script.scratch_path / 'constraints.txt', to_install + '[baz]',
+        allow_stderr_warning=True,
         expect_error=use_new_resolver
     )
     if use_new_resolver: