diff --git a/changelog.d/8432.misc b/changelog.d/8432.misc
new file mode 100644
index 000000000000..01fdad4caf6a
--- /dev/null
+++ b/changelog.d/8432.misc
@@ -0,0 +1 @@
+Check for unreachable code with mypy.
diff --git a/mypy.ini b/mypy.ini
index 79867814328b..c283f15b21e3 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -6,6 +6,7 @@ check_untyped_defs = True
 show_error_codes = True
 show_traceback = True
 mypy_path = stubs
+warn_unreachable = True
 files =
   synapse/api,
   synapse/appservice,
diff --git a/synapse/config/tls.py b/synapse/config/tls.py
index 9ddb8b546bad..ad37b93c0252 100644
--- a/synapse/config/tls.py
+++ b/synapse/config/tls.py
@@ -18,7 +18,7 @@
 import warnings
 from datetime import datetime
 from hashlib import sha256
-from typing import List
+from typing import List, Optional
 
 from unpaddedbase64 import encode_base64
 
@@ -177,8 +177,8 @@ def read_config(self, config: dict, config_dir_path: str, **kwargs):
             "use_insecure_ssl_client_just_for_testing_do_not_use"
         )
 
-        self.tls_certificate = None
-        self.tls_private_key = None
+        self.tls_certificate = None  # type: Optional[crypto.X509]
+        self.tls_private_key = None  # type: Optional[crypto.PKey]
 
     def is_disk_cert_valid(self, allow_self_signed=True):
         """
@@ -226,12 +226,12 @@ def is_disk_cert_valid(self, allow_self_signed=True):
         days_remaining = (expires_on - now).days
         return days_remaining
 
-    def read_certificate_from_disk(self, require_cert_and_key):
+    def read_certificate_from_disk(self, require_cert_and_key: bool):
         """
         Read the certificates and private key from disk.
 
         Args:
-            require_cert_and_key (bool): set to True to throw an error if the certificate
+            require_cert_and_key: set to True to throw an error if the certificate
                 and key file are not given
         """
         if require_cert_and_key:
@@ -479,13 +479,13 @@ def generate_config_section(
             }
         )
 
-    def read_tls_certificate(self):
+    def read_tls_certificate(self) -> crypto.X509:
         """Reads the TLS certificate from the configured file, and returns it
 
         Also checks if it is self-signed, and warns if so
 
         Returns:
-            OpenSSL.crypto.X509: the certificate
+            The certificate
         """
         cert_path = self.tls_certificate_file
         logger.info("Loading TLS certificate from %s", cert_path)
@@ -504,11 +504,11 @@ def read_tls_certificate(self):
 
         return cert
 
-    def read_tls_private_key(self):
+    def read_tls_private_key(self) -> crypto.PKey:
         """Reads the TLS private key from the configured file, and returns it
 
         Returns:
-            OpenSSL.crypto.PKey: the private key
+            The private key
         """
         private_key_path = self.tls_private_key_file
         logger.info("Loading TLS key from %s", private_key_path)
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 24329dd0e32f..02f11e120997 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -22,7 +22,6 @@
     Callable,
     Dict,
     List,
-    Match,
     Optional,
     Tuple,
     Union,
@@ -825,14 +824,14 @@ def server_matches_acl_event(server_name: str, acl_event: EventBase) -> bool:
     return False
 
 
-def _acl_entry_matches(server_name: str, acl_entry: str) -> Match:
+def _acl_entry_matches(server_name: str, acl_entry: Any) -> bool:
     if not isinstance(acl_entry, str):
         logger.warning(
             "Ignoring non-str ACL entry '%s' (is %s)", acl_entry, type(acl_entry)
         )
         return False
     regex = glob_to_regex(acl_entry)
-    return regex.match(server_name)
+    return bool(regex.match(server_name))
 
 
 class FederationHandlerRegistry:
diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index 62aa9a2da8f2..6f15c682405c 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -383,7 +383,7 @@ async def _user_can_delete_alias(self, alias: RoomAlias, user_id: str):
         """
         creator = await self.store.get_room_alias_creator(alias.to_string())
 
-        if creator is not None and creator == user_id:
+        if creator == user_id:
             return True
 
         # Resolve the alias to the corresponding room.
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index d5f7c78edf52..f1a6699cd447 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -962,8 +962,6 @@ async def _generate_room_id(
             try:
                 random_string = stringutils.random_string(18)
                 gen_room_id = RoomID(random_string, self.hs.hostname).to_string()
-                if isinstance(gen_room_id, bytes):
-                    gen_room_id = gen_room_id.decode("utf-8")
                 await self.store.store_room(
                     room_id=gen_room_id,
                     room_creator_user_id=creator_id,
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 8feba8c90a39..5ec36f591d78 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -642,7 +642,7 @@ async def copy_user_state_on_room_upgrade(
 
     async def send_membership_event(
         self,
-        requester: Requester,
+        requester: Optional[Requester],
         event: EventBase,
         context: EventContext,
         ratelimit: bool = True,
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index bfe2583002db..260ec19b4169 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -87,7 +87,7 @@ class SyncConfig:
 class TimelineBatch:
     prev_batch = attr.ib(type=StreamToken)
     events = attr.ib(type=List[EventBase])
-    limited = attr.ib(bool)
+    limited = attr.ib(type=bool)
 
     def __bool__(self) -> bool:
         """Make the result appear empty if there are no updates. This is used
diff --git a/synapse/http/server.py b/synapse/http/server.py
index 996a31a9ec0e..09ed74f6ce06 100644
--- a/synapse/http/server.py
+++ b/synapse/http/server.py
@@ -257,7 +257,7 @@ async def _async_render(self, request: Request):
             if isinstance(raw_callback_return, (defer.Deferred, types.CoroutineType)):
                 callback_return = await raw_callback_return
             else:
-                callback_return = raw_callback_return
+                callback_return = raw_callback_return  # type: ignore
 
             return callback_return
 
@@ -406,7 +406,7 @@ async def _async_render(self, request):
         if isinstance(raw_callback_return, (defer.Deferred, types.CoroutineType)):
             callback_return = await raw_callback_return
         else:
-            callback_return = raw_callback_return
+            callback_return = raw_callback_return  # type: ignore
 
         return callback_return
 
diff --git a/synapse/logging/_structured.py b/synapse/logging/_structured.py
index 144506c8f243..0fc2ea609e7e 100644
--- a/synapse/logging/_structured.py
+++ b/synapse/logging/_structured.py
@@ -12,7 +12,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
 import logging
 import os.path
 import sys
@@ -89,14 +88,7 @@ def __call__(self, event: dict) -> None:
         context = current_context()
 
         # Copy the context information to the log event.
-        if context is not None:
-            context.copy_to_twisted_log_entry(event)
-        else:
-            # If there's no logging context, not even the root one, we might be
-            # starting up or it might be from non-Synapse code. Log it as if it
-            # came from the root logger.
-            event["request"] = None
-            event["scope"] = None
+        context.copy_to_twisted_log_entry(event)
 
         self.observer(event)
 
diff --git a/synapse/push/push_rule_evaluator.py b/synapse/push/push_rule_evaluator.py
index 709ace01e5b8..3a68ce636fac 100644
--- a/synapse/push/push_rule_evaluator.py
+++ b/synapse/push/push_rule_evaluator.py
@@ -16,7 +16,7 @@
 
 import logging
 import re
-from typing import Any, Dict, List, Pattern, Union
+from typing import Any, Dict, List, Optional, Pattern, Union
 
 from synapse.events import EventBase
 from synapse.types import UserID
@@ -181,7 +181,7 @@ def _contains_display_name(self, display_name: str) -> bool:
 
         return r.search(body)
 
-    def _get_value(self, dotted_key: str) -> str:
+    def _get_value(self, dotted_key: str) -> Optional[str]:
         return self._value_cache.get(dotted_key, None)
 
 
diff --git a/synapse/replication/tcp/protocol.py b/synapse/replication/tcp/protocol.py
index 0b0d204e64bc..a509e599c20e 100644
--- a/synapse/replication/tcp/protocol.py
+++ b/synapse/replication/tcp/protocol.py
@@ -51,10 +51,11 @@
 import logging
 import struct
 from inspect import isawaitable
-from typing import TYPE_CHECKING, List
+from typing import TYPE_CHECKING, List, Optional
 
 from prometheus_client import Counter
 
+from twisted.internet import task
 from twisted.protocols.basic import LineOnlyReceiver
 from twisted.python.failure import Failure
 
@@ -152,9 +153,10 @@ def __init__(self, clock: Clock, handler: "ReplicationCommandHandler"):
 
         self.last_received_command = self.clock.time_msec()
         self.last_sent_command = 0
-        self.time_we_closed = None  # When we requested the connection be closed
+        # When we requested the connection be closed
+        self.time_we_closed = None  # type: Optional[int]
 
-        self.received_ping = False  # Have we reecived a ping from the other side
+        self.received_ping = False  # Have we received a ping from the other side
 
         self.state = ConnectionStates.CONNECTING
 
@@ -165,7 +167,7 @@ def __init__(self, clock: Clock, handler: "ReplicationCommandHandler"):
         self.pending_commands = []  # type: List[Command]
 
         # The LoopingCall for sending pings.
-        self._send_ping_loop = None
+        self._send_ping_loop = None  # type: Optional[task.LoopingCall]
 
         # a logcontext which we use for processing incoming commands. We declare it as a
         # background process so that the CPU stats get reported to prometheus.
diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py
index 31082bb16a5d..5b0900aa3cb0 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -738,7 +738,7 @@ def _make_state_cache_entry(
 
     # failing that, look for the closest match.
     prev_group = None
-    delta_ids = None
+    delta_ids = None  # type: Optional[StateMap[str]]
 
     for old_group, old_state in state_groups_ids.items():
         n_delta_ids = {k: v for k, v in new_state.items() if old_state.get(k) != v}
diff --git a/synapse/storage/databases/main/censor_events.py b/synapse/storage/databases/main/censor_events.py
index f211ddbaf88e..4bb2b9c28c8e 100644
--- a/synapse/storage/databases/main/censor_events.py
+++ b/synapse/storage/databases/main/censor_events.py
@@ -21,8 +21,8 @@
 from synapse.storage._base import SQLBaseStore
 from synapse.storage.database import DatabasePool
 from synapse.storage.databases.main.cache import CacheInvalidationWorkerStore
-from synapse.storage.databases.main.events import encode_json
 from synapse.storage.databases.main.events_worker import EventsWorkerStore
+from synapse.util.frozenutils import frozendict_json_encoder
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -105,7 +105,7 @@ async def _censor_redactions(self):
                 and original_event.internal_metadata.is_redacted()
             ):
                 # Redaction was allowed
-                pruned_json = encode_json(
+                pruned_json = frozendict_json_encoder.encode(
                     prune_event_dict(
                         original_event.room_version, original_event.get_dict()
                     )
@@ -171,7 +171,7 @@ def delete_expired_event_txn(txn):
                 return
 
             # Prune the event's dict then convert it to JSON.
-            pruned_json = encode_json(
+            pruned_json = frozendict_json_encoder.encode(
                 prune_event_dict(event.room_version, event.get_dict())
             )
 
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 18def01f5041..78e645592f6d 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -52,16 +52,6 @@
 )
 
 
-def encode_json(json_object):
-    """
-    Encode a Python object as JSON and return it in a Unicode string.
-    """
-    out = frozendict_json_encoder.encode(json_object)
-    if isinstance(out, bytes):
-        out = out.decode("utf8")
-    return out
-
-
 _EventCacheEntry = namedtuple("_EventCacheEntry", ("event", "redacted_event"))
 
 
@@ -743,7 +733,9 @@ def _update_outliers_txn(self, txn, events_and_contexts):
                     logger.exception("")
                     raise
 
-                metadata_json = encode_json(event.internal_metadata.get_dict())
+                metadata_json = frozendict_json_encoder.encode(
+                    event.internal_metadata.get_dict()
+                )
 
                 sql = "UPDATE event_json SET internal_metadata = ? WHERE event_id = ?"
                 txn.execute(sql, (metadata_json, event.event_id))
@@ -797,10 +789,10 @@ def event_dict(event):
                 {
                     "event_id": event.event_id,
                     "room_id": event.room_id,
-                    "internal_metadata": encode_json(
+                    "internal_metadata": frozendict_json_encoder.encode(
                         event.internal_metadata.get_dict()
                     ),
-                    "json": encode_json(event_dict(event)),
+                    "json": frozendict_json_encoder.encode(event_dict(event)),
                     "format_version": event.format_version,
                 }
                 for event, _ in events_and_contexts
diff --git a/synapse/storage/databases/main/stream.py b/synapse/storage/databases/main/stream.py
index 37249f1e3f99..1d27439536b7 100644
--- a/synapse/storage/databases/main/stream.py
+++ b/synapse/storage/databases/main/stream.py
@@ -546,7 +546,7 @@ async def get_recent_event_ids_for_room(
 
     async def get_room_event_before_stream_ordering(
         self, room_id: str, stream_ordering: int
-    ) -> Tuple[int, int, str]:
+    ) -> Optional[Tuple[int, int, str]]:
         """Gets details of the first event in a room at or before a stream ordering
 
         Args:
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 02fbb656e81c..ec356b2e4f06 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -421,7 +421,7 @@ def _mark_id_as_finished(self, next_id: int):
             self._unfinished_ids.discard(next_id)
             self._finished_ids.add(next_id)
 
-            new_cur = None
+            new_cur = None  # type: Optional[int]
 
             if self._unfinished_ids:
                 # If there are unfinished IDs then the new position will be the