Add type annotation to `visited_chains` (#17125)

This should fix CI on `develop`. Broke in
https://github.com/element-hq/synapse/commit/0fe9e1f7dafa80f3e02762f7ae75cefee5b3316c,
presumably due to a `mypy` dependency upgrade.

2 files changed, 2 insertions, 1 deletions

diff --git a/changelog.d/17125.misc b/changelog.d/17125.misc
new file mode 100644

index 0000000000..a7d9ce6491
--- /dev/null
+++ b/changelog.d/17125.misc
@@ -0,0 +1 @@
+Fix type annotation for `visited_chains` after `mypy` upgrade.
\ No newline at end of file
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py

index 1e731d56bd..990698aa5c 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -2454,7 +2454,7 @@ class _LinkMap:
             return target_seq <= src_seq
 
         # We have to graph traverse the links to check for indirect paths.
-        visited_chains = collections.Counter()
+        visited_chains: Dict[int, int] = collections.Counter()
         search = [(src_chain, src_seq)]
         while search:
             chain, seq = search.pop()