diff --git a/src/_pytest/assertion/__init__.py b/src/_pytest/assertion/__init__.py
index 0a57f6afa..ee7fa6a3a 100644
--- a/src/_pytest/assertion/__init__.py
+++ b/src/_pytest/assertion/__init__.py
@@ -2,6 +2,8 @@
 support for presenting detailed information in failing assertions.
 """
 import sys
+from typing import Any
+from typing import List
 from typing import Optional
 
 from _pytest.assertion import rewrite
@@ -9,6 +11,7 @@ from _pytest.assertion import truncate
 from _pytest.assertion import util
 from _pytest.assertion.rewrite import assertstate_key
 from _pytest.compat import TYPE_CHECKING
+from _pytest.config import Config
 from _pytest.config import hookimpl
 
 if TYPE_CHECKING:
@@ -170,5 +173,7 @@ def pytest_sessionfinish(session):
             assertstate.hook.set_session(None)
 
 
-def pytest_assertrepr_compare(config, op, left, right):
+def pytest_assertrepr_compare(
+    config: Config, op: str, left: Any, right: Any
+) -> Optional[List[str]]:
     return util.assertrepr_compare(config=config, op=op, left=left, right=right)
diff --git a/testing/test_assertion.py b/testing/test_assertion.py
index b12b3b119..3ce0f93e6 100644
--- a/testing/test_assertion.py
+++ b/testing/test_assertion.py
@@ -1,6 +1,9 @@
 import collections.abc as collections_abc
 import sys
 import textwrap
+from typing import Any
+from typing import List
+from typing import Optional
 
 import attr
 
@@ -310,9 +313,13 @@ class TestBinReprIntegration:
         result.stdout.fnmatch_lines(["*test_hello*FAIL*", "*test_check*PASS*"])
 
 
-def callequal(left, right, verbose=0):
+def callop(op: str, left: Any, right: Any, verbose: int = 0) -> Optional[List[str]]:
     config = mock_config(verbose=verbose)
-    return plugin.pytest_assertrepr_compare(config, "==", left, right)
+    return plugin.pytest_assertrepr_compare(config, op, left, right)
+
+
+def callequal(left: Any, right: Any, verbose: int = 0) -> Optional[List[str]]:
+    return callop("==", left, right, verbose)
 
 
 class TestAssert_reprcompare:
@@ -1068,10 +1075,13 @@ def test_rewritten(testdir):
     assert testdir.runpytest().ret == 0
 
 
-def test_reprcompare_notin():
-    config = mock_config()
-    detail = plugin.pytest_assertrepr_compare(config, "not in", "foo", "aaafoobbb")[1:]
-    assert detail == ["'foo' is contained here:", "  aaafoobbb", "?    +++"]
+def test_reprcompare_notin() -> None:
+    assert callop("not in", "foo", "aaafoobbb") == [
+        "'foo' not in 'aaafoobbb'",
+        "'foo' is contained here:",
+        "  aaafoobbb",
+        "?    +++",
+    ]
 
 
 def test_reprcompare_whitespaces():