diff --git a/changelog.d/15659.misc b/changelog.d/15659.misc
new file mode 100644
index 000000000000..93ceaeafc9b9
--- /dev/null
+++ b/changelog.d/15659.misc
@@ -0,0 +1 @@
+Improve type hints.
diff --git a/mypy.ini b/mypy.ini
index 3363c6daeee7..a7ec66196d41 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -32,9 +32,6 @@ warn_unused_ignores = False
 [mypy-synapse.util.caches.treecache]
 disallow_untyped_defs = False
 
-[mypy-tests.util.caches.test_descriptors]
-disallow_untyped_defs = False
-
 ;; Dependencies without annotations
 ;; Before ignoring a module, check to see if type stubs are available.
 ;; The `typeshed` project maintains stubs here:
diff --git a/tests/util/caches/test_descriptors.py b/tests/util/caches/test_descriptors.py
index 1a11341467ac..a4c1d3853b5b 100644
--- a/tests/util/caches/test_descriptors.py
+++ b/tests/util/caches/test_descriptors.py
@@ -13,7 +13,18 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import Iterable, Set, Tuple, cast
+from typing import (
+    Any,
+    Dict,
+    Generator,
+    Iterable,
+    List,
+    NoReturn,
+    Optional,
+    Set,
+    Tuple,
+    cast,
+)
 from unittest import mock
 
 from twisted.internet import defer, reactor
@@ -29,7 +40,7 @@
     make_deferred_yieldable,
 )
 from synapse.util.caches import descriptors
-from synapse.util.caches.descriptors import cached, cachedList
+from synapse.util.caches.descriptors import _CacheContext, cached, cachedList
 
 from tests import unittest
 from tests.test_utils import get_awaitable_result
@@ -37,21 +48,21 @@
 logger = logging.getLogger(__name__)
 
 
-def run_on_reactor():
-    d: "Deferred[int]" = defer.Deferred()
+def run_on_reactor() -> "Deferred[int]":
+    d: "Deferred[int]" = Deferred()
     cast(IReactorTime, reactor).callLater(0, d.callback, 0)
     return make_deferred_yieldable(d)
 
 
 class DescriptorTestCase(unittest.TestCase):
     @defer.inlineCallbacks
-    def test_cache(self):
+    def test_cache(self) -> Generator["Deferred[Any]", object, None]:
         class Cls:
             def __init__(self) -> None:
                 self.mock = mock.Mock()
 
             @descriptors.cached()
-            def fn(self, arg1, arg2):
+            def fn(self, arg1: int, arg2: int) -> str:
                 return self.mock(arg1, arg2)
 
         obj = Cls()
@@ -77,7 +88,7 @@ def fn(self, arg1, arg2):
         obj.mock.assert_not_called()
 
     @defer.inlineCallbacks
-    def test_cache_num_args(self):
+    def test_cache_num_args(self) -> Generator["Deferred[Any]", object, None]:
         """Only the first num_args arguments should matter to the cache"""
 
         class Cls:
@@ -85,7 +96,7 @@ def __init__(self) -> None:
                 self.mock = mock.Mock()
 
             @descriptors.cached(num_args=1)
-            def fn(self, arg1, arg2):
+            def fn(self, arg1: int, arg2: int) -> mock.Mock:
                 return self.mock(arg1, arg2)
 
         obj = Cls()
@@ -111,7 +122,7 @@ def fn(self, arg1, arg2):
         obj.mock.assert_not_called()
 
     @defer.inlineCallbacks
-    def test_cache_uncached_args(self):
+    def test_cache_uncached_args(self) -> Generator["Deferred[Any]", object, None]:
         """
         Only the arguments not named in uncached_args should matter to the cache
 
@@ -123,7 +134,7 @@ class Cls:
             # Note that it is important that this is not the last argument to
             # test behaviour of skipping arguments properly.
             @descriptors.cached(uncached_args=("arg2",))
-            def fn(self, arg1, arg2, arg3):
+            def fn(self, arg1: int, arg2: int, arg3: int) -> str:
                 return self.mock(arg1, arg2, arg3)
 
             def __init__(self) -> None:
@@ -152,7 +163,7 @@ def __init__(self) -> None:
         obj.mock.assert_not_called()
 
     @defer.inlineCallbacks
-    def test_cache_kwargs(self):
+    def test_cache_kwargs(self) -> Generator["Deferred[Any]", object, None]:
         """Test that keyword arguments are treated properly"""
 
         class Cls:
@@ -160,7 +171,7 @@ def __init__(self) -> None:
                 self.mock = mock.Mock()
 
             @descriptors.cached()
-            def fn(self, arg1, kwarg1=2):
+            def fn(self, arg1: int, kwarg1: int = 2) -> str:
                 return self.mock(arg1, kwarg1=kwarg1)
 
         obj = Cls()
@@ -193,7 +204,7 @@ def test_cache_with_sync_exception(self) -> None:
 
         class Cls:
             @cached()
-            def fn(self, arg1):
+            def fn(self, arg1: int) -> NoReturn:
                 raise SynapseError(100, "mai spoon iz too big!!1")
 
         obj = Cls()
@@ -213,11 +224,11 @@ def test_cache_with_async_exception(self) -> None:
         """The wrapped function returns a failure"""
 
         class Cls:
-            result = None
+            result: Optional[Deferred] = None
             call_count = 0
 
             @cached()
-            def fn(self, arg1):
+            def fn(self, arg1: int) -> Optional[Deferred]:
                 self.call_count += 1
                 return self.result
 
@@ -225,7 +236,7 @@ def fn(self, arg1):
         callbacks: Set[str] = set()
 
         # set off an asynchronous request
-        origin_d: Deferred = defer.Deferred()
+        origin_d: Deferred = Deferred()
         obj.result = origin_d
 
         d1 = obj.fn(1, on_invalidate=lambda: callbacks.add("d1"))
@@ -260,17 +271,17 @@ def fn(self, arg1):
         self.assertEqual(self.successResultOf(d3), 100)
         self.assertEqual(obj.call_count, 2)
 
-    def test_cache_logcontexts(self):
+    def test_cache_logcontexts(self) -> Deferred:
         """Check that logcontexts are set and restored correctly when
         using the cache."""
 
-        complete_lookup: Deferred = defer.Deferred()
+        complete_lookup: Deferred = Deferred()
 
         class Cls:
             @descriptors.cached()
-            def fn(self, arg1):
+            def fn(self, arg1: int) -> Deferred[int]:
                 @defer.inlineCallbacks
-                def inner_fn():
+                def inner_fn() -> Generator["Deferred[object]", object, int]:
                     with PreserveLoggingContext():
                         yield complete_lookup
                     return 1
@@ -278,13 +289,13 @@ def inner_fn():
                 return inner_fn()
 
         @defer.inlineCallbacks
-        def do_lookup():
+        def do_lookup() -> Generator["Deferred[Any]", object, int]:
             with LoggingContext("c1") as c1:
                 r = yield obj.fn(1)
                 self.assertEqual(current_context(), c1)
-            return r
+            return cast(int, r)
 
-        def check_result(r):
+        def check_result(r: int) -> None:
             self.assertEqual(r, 1)
 
         obj = Cls()
@@ -304,15 +315,15 @@ def check_result(r):
 
         return defer.gatherResults([d1, d2])
 
-    def test_cache_logcontexts_with_exception(self):
+    def test_cache_logcontexts_with_exception(self) -> Deferred:
         """Check that the cache sets and restores logcontexts correctly when
         the lookup function throws an exception"""
 
         class Cls:
             @descriptors.cached()
-            def fn(self, arg1):
+            def fn(self, arg1: int) -> Deferred:
                 @defer.inlineCallbacks
-                def inner_fn():
+                def inner_fn() -> Generator["Deferred[Any]", object, NoReturn]:
                     # we want this to behave like an asynchronous function
                     yield run_on_reactor()
                     raise SynapseError(400, "blah")
@@ -320,7 +331,7 @@ def inner_fn():
                 return inner_fn()
 
         @defer.inlineCallbacks
-        def do_lookup():
+        def do_lookup() -> Generator["Deferred[object]", object, None]:
             with LoggingContext("c1") as c1:
                 try:
                     d = obj.fn(1)
@@ -347,13 +358,13 @@ def do_lookup():
         return d1
 
     @defer.inlineCallbacks
-    def test_cache_default_args(self):
+    def test_cache_default_args(self) -> Generator["Deferred[Any]", object, None]:
         class Cls:
             def __init__(self) -> None:
                 self.mock = mock.Mock()
 
             @descriptors.cached()
-            def fn(self, arg1, arg2=2, arg3=3):
+            def fn(self, arg1: int, arg2: int = 2, arg3: int = 3) -> str:
                 return self.mock(arg1, arg2, arg3)
 
         obj = Cls()
@@ -384,13 +395,13 @@ def fn(self, arg1, arg2=2, arg3=3):
         self.assertEqual(r, "chips")
         obj.mock.assert_not_called()
 
-    def test_cache_iterable(self):
+    def test_cache_iterable(self) -> None:
         class Cls:
             def __init__(self) -> None:
                 self.mock = mock.Mock()
 
             @descriptors.cached(iterable=True)
-            def fn(self, arg1, arg2):
+            def fn(self, arg1: int, arg2: int) -> List[str]:
                 return self.mock(arg1, arg2)
 
         obj = Cls()
@@ -422,7 +433,7 @@ def test_cache_iterable_with_sync_exception(self) -> None:
 
         class Cls:
             @descriptors.cached(iterable=True)
-            def fn(self, arg1):
+            def fn(self, arg1: int) -> NoReturn:
                 raise SynapseError(100, "mai spoon iz too big!!1")
 
         obj = Cls()
@@ -443,15 +454,15 @@ def test_invalidate_cascade(self) -> None:
 
         class Cls:
             @cached(cache_context=True)
-            async def func1(self, key, cache_context):
+            async def func1(self, key: str, cache_context: _CacheContext) -> int:
                 return await self.func2(key, on_invalidate=cache_context.invalidate)
 
             @cached(cache_context=True)
-            async def func2(self, key, cache_context):
+            async def func2(self, key: str, cache_context: _CacheContext) -> int:
                 return await self.func3(key, on_invalidate=cache_context.invalidate)
 
             @cached(cache_context=True)
-            async def func3(self, key, cache_context):
+            async def func3(self, key: str, cache_context: _CacheContext) -> int:
                 self.invalidate = cache_context.invalidate
                 return 42
 
@@ -469,7 +480,7 @@ def test_cancel(self) -> None:
 
         class Cls:
             @cached()
-            async def fn(self, arg1):
+            async def fn(self, arg1: int) -> str:
                 await complete_lookup
                 return str(arg1)
 
@@ -501,7 +512,7 @@ class Cls:
             inner_context_was_finished = False
 
             @cached()
-            async def fn(self, arg1):
+            async def fn(self, arg1: int) -> str:
                 await make_deferred_yieldable(complete_lookup)
                 self.inner_context_was_finished = current_context().finished
                 return str(arg1)
@@ -542,10 +553,10 @@ class CacheDecoratorTestCase(unittest.HomeserverTestCase):
     """
 
     @defer.inlineCallbacks
-    def test_passthrough(self):
+    def test_passthrough(self) -> Generator["Deferred[Any]", object, None]:
         class A:
             @cached()
-            def func(self, key):
+            def func(self, key: str) -> str:
                 return key
 
         a = A()
@@ -554,12 +565,12 @@ def func(self, key):
         self.assertEqual((yield a.func("bar")), "bar")
 
     @defer.inlineCallbacks
-    def test_hit(self):
+    def test_hit(self) -> Generator["Deferred[Any]", object, None]:
         callcount = [0]
 
         class A:
             @cached()
-            def func(self, key):
+            def func(self, key: str) -> str:
                 callcount[0] += 1
                 return key
 
@@ -572,12 +583,12 @@ def func(self, key):
         self.assertEqual(callcount[0], 1)
 
     @defer.inlineCallbacks
-    def test_invalidate(self):
+    def test_invalidate(self) -> Generator["Deferred[Any]", object, None]:
         callcount = [0]
 
         class A:
             @cached()
-            def func(self, key):
+            def func(self, key: str) -> str:
                 callcount[0] += 1
                 return key
 
@@ -592,21 +603,21 @@ def func(self, key):
 
         self.assertEqual(callcount[0], 2)
 
-    def test_invalidate_missing(self):
+    def test_invalidate_missing(self) -> None:
         class A:
             @cached()
-            def func(self, key):
+            def func(self, key: str) -> str:
                 return key
 
         A().func.invalidate(("what",))
 
     @defer.inlineCallbacks
-    def test_max_entries(self):
+    def test_max_entries(self) -> Generator["Deferred[Any]", object, None]:
         callcount = [0]
 
         class A:
             @cached(max_entries=10)
-            def func(self, key):
+            def func(self, key: int) -> int:
                 callcount[0] += 1
                 return key
 
@@ -626,14 +637,14 @@ def func(self, key):
             callcount[0] >= 14, msg="Expected callcount >= 14, got %d" % (callcount[0])
         )
 
-    def test_prefill(self):
+    def test_prefill(self) -> None:
         callcount = [0]
 
         d = defer.succeed(123)
 
         class A:
             @cached()
-            def func(self, key):
+            def func(self, key: str) -> Deferred[int]:
                 callcount[0] += 1
                 return d
 
@@ -645,18 +656,18 @@ def func(self, key):
         self.assertEqual(callcount[0], 0)
 
     @defer.inlineCallbacks
-    def test_invalidate_context(self):
+    def test_invalidate_context(self) -> Generator["Deferred[Any]", object, None]:
         callcount = [0]
         callcount2 = [0]
 
         class A:
             @cached()
-            def func(self, key):
+            def func(self, key: str) -> str:
                 callcount[0] += 1
                 return key
 
             @cached(cache_context=True)
-            def func2(self, key, cache_context):
+            def func2(self, key: str, cache_context: _CacheContext) -> "Deferred[str]":
                 callcount2[0] += 1
                 return self.func(key, on_invalidate=cache_context.invalidate)
 
@@ -678,18 +689,18 @@ def func2(self, key, cache_context):
         self.assertEqual(callcount2[0], 2)
 
     @defer.inlineCallbacks
-    def test_eviction_context(self):
+    def test_eviction_context(self) -> Generator["Deferred[Any]", object, None]:
         callcount = [0]
         callcount2 = [0]
 
         class A:
             @cached(max_entries=2)
-            def func(self, key):
+            def func(self, key: str) -> str:
                 callcount[0] += 1
                 return key
 
             @cached(cache_context=True)
-            def func2(self, key, cache_context):
+            def func2(self, key: str, cache_context: _CacheContext) -> "Deferred[str]":
                 callcount2[0] += 1
                 return self.func(key, on_invalidate=cache_context.invalidate)
 
@@ -715,18 +726,18 @@ def func2(self, key, cache_context):
         self.assertEqual(callcount2[0], 3)
 
     @defer.inlineCallbacks
-    def test_double_get(self):
+    def test_double_get(self) -> Generator["Deferred[Any]", object, None]:
         callcount = [0]
         callcount2 = [0]
 
         class A:
             @cached()
-            def func(self, key):
+            def func(self, key: str) -> str:
                 callcount[0] += 1
                 return key
 
             @cached(cache_context=True)
-            def func2(self, key, cache_context):
+            def func2(self, key: str, cache_context: _CacheContext) -> "Deferred[str]":
                 callcount2[0] += 1
                 return self.func(key, on_invalidate=cache_context.invalidate)
 
@@ -763,17 +774,17 @@ def func2(self, key, cache_context):
 
 class CachedListDescriptorTestCase(unittest.TestCase):
     @defer.inlineCallbacks
-    def test_cache(self):
+    def test_cache(self) -> Generator["Deferred[Any]", object, None]:
         class Cls:
             def __init__(self) -> None:
                 self.mock = mock.Mock()
 
             @descriptors.cached()
-            def fn(self, arg1, arg2):
+            def fn(self, arg1: int, arg2: int) -> None:
                 pass
 
             @descriptors.cachedList(cached_method_name="fn", list_name="args1")
-            async def list_fn(self, args1, arg2):
+            async def list_fn(self, args1: Iterable[int], arg2: int) -> Dict[int, str]:
                 context = current_context()
                 assert isinstance(context, LoggingContext)
                 assert context.name == "c1"
@@ -824,7 +835,7 @@ async def list_fn(self, args1, arg2):
             obj.mock.assert_called_once_with({40}, 2)
             self.assertEqual(r, {10: "fish", 40: "gravy"})
 
-    def test_concurrent_lookups(self):
+    def test_concurrent_lookups(self) -> None:
         """All concurrent lookups should get the same result"""
 
         class Cls:
@@ -832,11 +843,11 @@ def __init__(self) -> None:
                 self.mock = mock.Mock()
 
             @descriptors.cached()
-            def fn(self, arg1):
+            def fn(self, arg1: int) -> None:
                 pass
 
             @descriptors.cachedList(cached_method_name="fn", list_name="args1")
-            def list_fn(self, args1) -> "Deferred[dict]":
+            def list_fn(self, args1: List[int]) -> "Deferred[dict]":
                 return self.mock(args1)
 
         obj = Cls()
@@ -867,7 +878,7 @@ def list_fn(self, args1) -> "Deferred[dict]":
         self.assertEqual(self.successResultOf(d3), {10: "peas"})
 
     @defer.inlineCallbacks
-    def test_invalidate(self):
+    def test_invalidate(self) -> Generator["Deferred[Any]", object, None]:
         """Make sure that invalidation callbacks are called."""
 
         class Cls:
@@ -875,11 +886,11 @@ def __init__(self) -> None:
                 self.mock = mock.Mock()
 
             @descriptors.cached()
-            def fn(self, arg1, arg2):
+            def fn(self, arg1: int, arg2: int) -> None:
                 pass
 
             @descriptors.cachedList(cached_method_name="fn", list_name="args1")
-            async def list_fn(self, args1, arg2):
+            async def list_fn(self, args1: List[int], arg2: int) -> Dict[int, str]:
                 # we want this to behave like an asynchronous function
                 await run_on_reactor()
                 return self.mock(args1, arg2)
@@ -908,17 +919,17 @@ async def list_fn(self, args1, arg2):
         invalidate0.assert_called_once()
         invalidate1.assert_called_once()
 
-    def test_cancel(self):
+    def test_cancel(self) -> None:
         """Test that cancelling a lookup does not cancel other lookups"""
         complete_lookup: "Deferred[None]" = Deferred()
 
         class Cls:
             @cached()
-            def fn(self, arg1):
+            def fn(self, arg1: int) -> None:
                 pass
 
             @cachedList(cached_method_name="fn", list_name="args")
-            async def list_fn(self, args):
+            async def list_fn(self, args: List[int]) -> Dict[int, str]:
                 await complete_lookup
                 return {arg: str(arg) for arg in args}
 
@@ -936,7 +947,7 @@ async def list_fn(self, args):
         self.failureResultOf(d1, CancelledError)
         self.assertEqual(d2.result, {123: "123", 456: "456", 789: "789"})
 
-    def test_cancel_logcontexts(self):
+    def test_cancel_logcontexts(self) -> None:
         """Test that cancellation does not break logcontexts.
 
         * The `CancelledError` must be raised with the correct logcontext.
@@ -949,11 +960,11 @@ class Cls:
             inner_context_was_finished = False
 
             @cached()
-            def fn(self, arg1):
+            def fn(self, arg1: int) -> None:
                 pass
 
             @cachedList(cached_method_name="fn", list_name="args")
-            async def list_fn(self, args):
+            async def list_fn(self, args: List[int]) -> Dict[int, str]:
                 await make_deferred_yieldable(complete_lookup)
                 self.inner_context_was_finished = current_context().finished
                 return {arg: str(arg) for arg in args}
@@ -991,14 +1002,14 @@ def test_num_args_mismatch(self) -> None:
 
         class Cls:
             @descriptors.cached(tree=True)
-            def fn(self, room_id, event_id):
+            def fn(self, room_id: str, event_id: str) -> None:
                 pass
 
             # This is wrong ❌. `@cachedList` expects to be given the same number
             # of arguments as the underlying cached function, just with one of
             # the arguments being an iterable
             @descriptors.cachedList(cached_method_name="fn", list_name="keys")
-            def list_fn(self, keys: Iterable[Tuple[str, str]]):
+            def list_fn(self, keys: Iterable[Tuple[str, str]]) -> None:
                 pass
 
             # Corrected syntax ✅