Crash when type variable has same name as an external symbol with an erroneous definition in PEP 695 generic class

Crash Report

This crashes on master:

T = NotDefined
class A[T]: ...

Does not reproduce when using a generic function (def f[T](): ...) or when T has some other non-type-variable value (T = 1)

Traceback

TESTFILES/SCRATCH.py:2: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.18.0+dev.ad76e1628e2d4fe52d89a80d51890bf84afad38f
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/brian/Projects/open-contrib/mypy/mypy/__main__.py", line 37, in <module>
    console_entry()
  File "/home/brian/Projects/open-contrib/mypy/mypy/__main__.py", line 15, in console_entry
    main()
  File "/home/brian/Projects/open-contrib/mypy/mypy/main.py", line 127, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/home/brian/Projects/open-contrib/mypy/mypy/main.py", line 211, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/brian/Projects/open-contrib/mypy/mypy/build.py", line 191, in build
    result = _build(
  File "/home/brian/Projects/open-contrib/mypy/mypy/build.py", line 267, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/brian/Projects/open-contrib/mypy/mypy/build.py", line 2939, in dispatch
    process_graph(graph, manager)
  File "/home/brian/Projects/open-contrib/mypy/mypy/build.py", line 3337, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/brian/Projects/open-contrib/mypy/mypy/build.py", line 3432, in process_stale_scc
    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal_main.py", line 95, in semantic_analysis_for_scc
    process_top_levels(graph, scc, patches)
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal_main.py", line 222, in process_top_levels
    deferred, incomplete, progress = semantic_analyze_target(
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal_main.py", line 399, in semantic_analyze_target
    analyzer.refresh_partial(
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal.py", line 652, in refresh_partial
    self.refresh_top_level(node)
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal.py", line 670, in refresh_top_level
    self.accept(d)
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal.py", line 7314, in accept
    node.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "/home/brian/Projects/open-contrib/mypy/mypy/nodes.py", line 1249, in accept
    return visitor.visit_class_def(self)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal.py", line 1769, in visit_class_def
    self.analyze_class(defn)
    ~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal.py", line 1910, in analyze_class
    bases, tvar_defs, is_protocol = self.clean_up_bases_and_infer_type_variables(
                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        defn, bases, context=defn
        ^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal.py", line 2215, in clean_up_bases_and_infer_type_variables
    assert isinstance(node.node, TypeVarLikeExpr)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError:

@ilevkivskyi

mypy/mypy/semanal.py

Lines 6779 to 6784 in ad76e16

    
           elif name not in self.missing_names[-1] and "*" not in self.missing_names[-1]: 
        
               names[name] = symbol 
        
               if not no_progress: 
        
                   self.progress = True 
        
               return True 
        
           return False

So T is a missing name, and we refuse it everywhere.

mypy/mypy/semanal.py

Lines 389 to 399 in ad76e16

    
           # These names couldn't be added to the symbol table due to incomplete deps. 
        
           # Note that missing names are per module, _not_ per namespace. This means that e.g. 
        
           # a missing name at global scope will block adding same name at a class scope. 
        
           # This should not affect correctness and is purely a performance issue, 
        
           # since it can cause unnecessary deferrals. These are represented as 
        
           # PlaceholderNodes in the symbol table. We use this to ensure that the first 
        
           # definition takes precedence even if it's incomplete. 
        
           # 
        
           # Note that a star import adds a special name '*' to the set, this blocks 
        
           # adding _any_ names in the current file. 
        
           missing_names: list[set[str]]

cc @ilevkivskyi - would it be ok to exclude type parameters from this restriction?

diff --git a/mypy/semanal.py b/mypy/semanal.py
index 1840e606a..aff5a7601 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -6776,7 +6776,9 @@ class SemanticAnalyzer(
                     self.add_redefinition(names, name, symbol)
                 if not (isinstance(new, (FuncDef, Decorator)) and self.set_original_def(old, new)):
                     self.name_already_defined(name, context, existing)
-        elif name not in self.missing_names[-1] and "*" not in self.missing_names[-1]:
+        elif type_param or (
+            name not in self.missing_names[-1] and "*" not in self.missing_names[-1]
+        ):
             names[name] = symbol
             if not no_progress:
                 self.progress = True

@JukkaL

I think @JukkaL may be a better person to ask, as IIRC he implemented most of PEP 695.

Allow type parameters reusing the name missing from current module (p…

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Crash when type variable has same name as an external symbol with an erroneous definition in PEP 695 generic class #19526

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	elif name not in self.missing_names[-1] and "*" not in self.missing_names[-1]:
	names[name] = symbol
	if not no_progress:
	self.progress = True
	return True
	return False

	# These names couldn't be added to the symbol table due to incomplete deps.
	# Note that missing names are per module, _not_ per namespace. This means that e.g.
	# a missing name at global scope will block adding same name at a class scope.
	# This should not affect correctness and is purely a performance issue,
	# since it can cause unnecessary deferrals. These are represented as
	# PlaceholderNodes in the symbol table. We use this to ensure that the first
	# definition takes precedence even if it's incomplete.
	#
	# Note that a star import adds a special name '*' to the set, this blocks
	# adding _any_ names in the current file.
	missing_names: list[set[str]]

Crash when type variable has same name as an external symbol with an erroneous definition in PEP 695 generic class #19526

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sterliakov commented on Jul 29, 2025

ilevkivskyi commented on Aug 3, 2025

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