diff --git a/src/pip/_internal/req/req_install.py b/src/pip/_internal/req/req_install.py
index 1d3ee5edc..7538c9546 100644
--- a/src/pip/_internal/req/req_install.py
+++ b/src/pip/_internal/req/req_install.py
@@ -864,3 +864,35 @@ class InstallRequirement(object):
             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
diff --git a/src/pip/_internal/resolution/resolvelib/resolver.py b/src/pip/_internal/resolution/resolvelib/resolver.py
index 68f06aa9e..841057135 100644
--- a/src/pip/_internal/resolution/resolvelib/resolver.py
+++ b/src/pip/_internal/resolution/resolvelib/resolver.py
@@ -7,10 +7,10 @@ from pip._vendor.resolvelib import BaseReporter, ResolutionImpossible
 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
@@ -32,38 +32,6 @@ if MYPY_CHECK_RUNNING:
 logger = logging.getLogger(__name__)
 
 
-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
-
-
 class Resolver(BaseResolver):
     _allowed_strategies = {"eager", "only-if-needed", "to-satisfy-only"}