Add tests for constraints

tests/functional/test_new_resolver.py

@@ -529,3 +529,54 @@ def test_new_resolver_handles_prerelease(
         *pip_args
     )
     assert_installed(script, pkg=expected_version)
+
+
+def test_new_resolver_constraints(script):
+    create_basic_wheel_for_package(script, "pkg", "1.0")
+    create_basic_wheel_for_package(script, "pkg", "2.0")
+    create_basic_wheel_for_package(script, "pkg", "3.0")
+    constraints_file = script.scratch_path / "constraints.txt"
+    constraints_file.write_text("pkg<2.0\nconstraint_only<1.0")
+    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")
+    assert_not_installed(script, "constraint_only")
+
+
+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")
+    constraints_file = script.scratch_path / "constraints.txt"
+    constraints_file.write_text("dep==1.0")
+    script.pip(
+        "install", "--unstable-feature=resolver",
+        "--no-cache-dir", "--no-index",
+        "--find-links", script.scratch_path,
+        "-c", constraints_file,
+        "base"
+    )
+    assert_installed(script, base="1.0")
+    assert_installed(script, dep="1.0")
+
+
+def test_new_resolver_constraint_on_path(script):
+    setup_py = script.scratch_path / "setup.py"
+    text = "from setuptools import setup\nsetup(name='foo', version='2.0')"
+    setup_py.write_text(text)
+    constraints_txt = script.scratch_path / "constraints.txt"
+    constraints_txt.write_text("foo==1.0")
+    result = script.pip(
+        "install", "--unstable-feature=resolver",
+        "--no-cache-dir", "--no-index",
+        "-c", constraints_txt,
+        str(script.scratch_path),
+        expect_error=True,
+    )
+
+    msg = "installation from path or url cannot be constrained to a version"
+    assert msg in result.stderr, str(result)