Description

dariushoule

opened on Aug 13, 2026

Issue body actions

Crash report

What happened?

types.GenericAlias calculates its __parameters__ only when they are first needed, then caches the result on the alias.

If an argument gains a __typing_subst__ attribute after the result has been cached, the cached parameters no longer match the alias arguments. During a later substitution, _Py_subs_parameters() recognizes the changed argument as a parameter but cannot find it in the cached tuple, so tuple_index() returns -1.

In a debug build, this triggers a failed C assertion. In a release build, the -1 is used as an array index and can crash the process.

class Parameter:
    pass

first = Parameter()
first.__typing_subst__ = lambda value: value
late = Parameter()

alias = dict[first, late]
alias.__parameters__
late.__typing_subst__ = lambda value: value
alias[0,]

CPython versions tested on:

3.15, 3.14, 3.13, 3.12, 3.11

Operating systems tested on:

macOS, Linux

Output from running 'python -VV' on the command line:

Python 3.14.6 (main, Jun 10 2026, 10:03:53) [Clang 21.0.0 (clang-2100.0.123.102)]

Linked PRs

• gh-155752: Do not crash when GenericAlias parameters change during substitution #155761
• [3.15] gh-155752: Do not crash when GenericAlias parameters change during substitution (GH-155761) #155770
• [3.14] gh-155752: Do not crash when GenericAlias parameters change during substitution (GH-155761) #155771
• [3.13] gh-155752: Do not crash when GenericAlias parameters change during substitution (GH-155761) #155772

Activity

1. dariushoule

   added

   type-crashA hard crash of the interpreter, possibly with a core dumpA hard crash of the interpreter, possibly with a core dump

   on Aug 13, 2026
2. dariushoule

   changed the title [-]GenericAlias parameter substitution can crash on a missed cached parameter[/-] [+]GenericAlias crashes when an argument gains __typing_subst__ after parameters are cached[/+] on Aug 13, 2026
3. JelleZijlstra

   added

   topic-typing

   on Aug 13, 2026
4. dariushoule

   added a commit that references this issue on Aug 13, 2026

   pythongh-155752: Do not crash when GenericAlias parameters change dur…

   7a1932e
5. picnixz

   added

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   on Aug 13, 2026
6. JelleZijlstra

   added a commit that references this issue on Aug 14, 2026

   gh-155752: Do not crash when GenericAlias parameters change during su…

   c0006fa
7. JelleZijlstra

   closed this as completedin #155761on Aug 14, 2026
8. hugovk

   added a commit that references this issue on Aug 14, 2026

   [3.15] gh-155752: Do not crash when GenericAlias parameters change du…

   b8a9987
9. mbeijen

   added a commit that references this issue on Aug 14, 2026

   pythongh-155752: Do not crash when GenericAlias parameters change dur…

   1772486
10. terryjreedy

    added 2 commits that reference this issue on Sep 8, 2026

    [3.14] gh-155752: Do not crash when GenericAlias parameters change du…

    96964c8

    [3.13] gh-155752: Do not crash when GenericAlias parameters change du…

    649e421

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