2.2 Regression with placeholder types in a TypeVar

**Crash Report** I was testing the [mypy 2.2 branch](https://github.com/python/mypy/issues/21618#issuecomment-4784328932) and I came across some errors around placeholder types. **Traceback** There's kinda two errors and I think they're related <details> <summary>AssertionError: Must not defer during final iteration</summary> ``` Please report a bug at https://github.com/python/mypy/issues version: 2.2.0+dev.5ef090270579013c337a9bcc79e2c58417f09538 Traceback (most recent call last): File "<frozen runpy>", line 198, in _run_module_as_main File "<frozen runpy>", line 88, in _run_code File "mypy/semanal.py", line 7732, in accept File "mypy/nodes.py", line 1165, in accept File "mypy/semanal.py", line 991, in visit_func_def File "mypy/semanal.py", line 1036, in analyze_func_def File "mypy/semanal.py", line 7333, in defer AssertionError: Must not defer during final iteration ``` </details> If I use the source directly so I get more of a traceback <details> <summary>More details</summary> ``` version: 2.2.0+dev.5ef090270579013c337a9bcc79e2c58417f09538 Traceback (most recent call last): File "<frozen runpy>", line 198, in _run_module_as_main File "<frozen runpy>", line 88, in _run_code File "/Users/stephen.moore/Projects/external/mypy/mypy/__main__.py", line 52, in <module> console_entry() File "/Users/stephen.moore/Projects/external/mypy/mypy/__main__.py", line 16, in console_entry main() File "/Users/stephen.moore/Projects/external/mypy/mypy/main.py", line 154, in main res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr) File "/Users/stephen.moore/Projects/external/mypy/mypy/main.py", line 244, in run_build res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr) File "/Users/stephen.moore/Projects/external/mypy/mypy/build.py", line 422, in build result = build_inner( File "/Users/stephen.moore/Projects/external/mypy/mypy/build.py", line 537, in build_inner graph = dispatch(sources, manager, stdout, connect_threads) File "/Users/stephen.moore/Projects/external/mypy/mypy/build.py", line 4146, in dispatch process_graph(graph, manager) File "/Users/stephen.moore/Projects/external/mypy/mypy/build.py", line 4614, in process_graph done, still_working, results = manager.wait_for_done(graph) File "/Users/stephen.moore/Projects/external/mypy/mypy/build.py", line 1484, in wait_for_done process_stale_scc(graph, next_scc, self) File "/Users/stephen.moore/Projects/external/mypy/mypy/build.py", line 4782, in process_stale_scc mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors) File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal_main.py", line 92, in semantic_analysis_for_scc process_functions(graph, scc, patches) File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal_main.py", line 275, in process_functions process_top_level_function( File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal_main.py", line 314, in process_top_level_function deferred, incomplete, progress = semantic_analyze_target( File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal_main.py", line 380, in semantic_analyze_target analyzer.refresh_partial( File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal.py", line 709, in refresh_partial self.accept(node) File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal.py", line 7732, in accept node.accept(self) ~~~~~~~~~~~^^^^^^ File "/Users/stephen.moore/Projects/external/mypy/mypy/nodes.py", line 1165, in accept return visitor.visit_func_def(self) ~~~~~~~~~~~~~~~~~~~~~~^^^^^^ File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal.py", line 991, in visit_func_def self.analyze_func_def(defn) ~~~~~~~~~~~~~~~~~~~~~^^^^^^ File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal.py", line 1036, in analyze_func_def self.defer(defn) ~~~~~~~~~~^^^^^^ File "/Users/stephen.moore/Projects/external/mypy/mypy/semanal.py", line 7333, in defer assert not self.final_iteration, "Must not defer during final iteration" ^^^^^^^^^^^^^^^^^^^^^^^^ AssertionError: Must not defer during final iteration ``` </details> And if I start trying to fill in default values I start to get this happening: <details> <summary>NotImplementedError: Cannot serialize PlaceholderType instance</summary> ``` Traceback (most recent call last): File "<frozen runpy>", line 198, in _run_module_as_main File "<frozen runpy>", line 88, in _run_code File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/__main__.py", line 52, in <module> console_entry() File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/__main__.py", line 16, in console_entry main() ~~~~^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/main.py", line 154, in main res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr) ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/main.py", line 244, in run_build res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr) File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/build.py", line 422, in build result = build_inner( sources, ...<9 lines>... metastore, ) File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/build.py", line 537, in build_inner graph = dispatch(sources, manager, stdout, connect_threads) File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/build.py", line 4146, in dispatch process_graph(graph, manager) ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/build.py", line 4614, in process_graph done, still_working, results = manager.wait_for_done(graph) ~~~~~~~~~~~~~~~~~~~~~^^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/build.py", line 1484, in wait_for_done process_stale_scc(graph, next_scc, self) ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/build.py", line 4821, in process_stale_scc meta_tuple = graph[id].write_cache() File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/build.py", line 3604, in write_cache new_interface_hash, meta_tuple = write_cache( ~~~~~~~~~~~^ self.id, ^^^^^^^^ ...<13 lines>... self.manager, ^^^^^^^^^^^^^ ) ^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/build.py", line 2325, in write_cache tree.write(data_io) ~~~~~~~~~~^^^^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/nodes.py", line 599, in write self.names.write(data, self._fullname) ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/nodes.py", line 5169, in write value.write(data, fullname, key) ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/nodes.py", line 5074, in write self.node.write(data) ~~~~~~~~~~~~~~~^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/nodes.py", line 4721, in write self.target.write(data) ~~~~~~~~~~~~~~~~~^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/types.py", line 1756, in write write_type_list(data, self.args) ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/types.py", line 4553, in write_type_list item.write(data) ~~~~~~~~~~^^^^^^ File "/Users/stephen.moore/.virtualenvs/project/lib/python3.13/site-packages/mypy/types.py", line 318, in write raise NotImplementedError(f"Cannot serialize {self.__class__.__name__} instance") NotImplementedError: Cannot serialize PlaceholderType instance ``` </details> **To Reproduce** It appears that ultimately this is an interaction with how my [mypy plugin](https://github.com/delfick/extended-mypy-django-plugin) works, which complicates it a little. Essentially I add dependencies with the `get_additional_deps` hook that contain aliases (I call these virtual dependencies), and then with a `get_type_analyze_hook` hook I look for a special class that I replace with the target of an appropriate alias from those "virtual dependencies". I have created an example with a readme that replicates the problem. Seems I can't upload a git bundle to a github issue, so here is a repo: https://github.com/delfick/mypy-release-2.2-bug-demonstration It seems there is one change required to fix it ```patch From 84140edcb76aaf4b6bf3a2fd1f6a5ffc80277810 Mon Sep 17 00:00:00 2001 From: Stephen Moore <stephen.moore@kraken.tech> Date: Wed, 24 Jun 2026 10:13:32 +1000 Subject: [PATCH] WIP: this makes a difference --- mypy/semanal.py | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mypy/semanal.py b/mypy/semanal.py index e010273b0..324910809 100644 --- a/mypy/semanal.py +++ b/mypy/semanal.py @@ -2009,7 +2009,7 @@ class SemanticAnalyzer( # re-analyze this class. Note we force progress to handle cases like # class C[T = C], this matches logic in process_typevar_parameters() # for "old style" type variables. - self.defer(force_progress=tvar_defs != defn.type_vars) + self.mark_incomplete(defn.name, defn) self.analyze_class_keywords(defn) bases_result = self.analyze_base_classes(defn.name, bases) -- 2.50.1 (Apple Git-155) ``` (undoing a change in https://github.com/python/mypy/pull/21491) However when I do this mypy unit tests start to fail. I can't figure out how to fix that, or even how to replicate the errors in a mypy unit test. **Your Environment** <!-- Include as many relevant details about the environment you experienced the bug in --> - Mypy version used: release-2.2 branch (5ef090270579013c337a9bcc79e2c58417f09538) - Python version used: 3.13.2 - Operating system and version: MacOS Tahoe 26.5.1 AI Disclosure: None: I actively avoid LLMs