Fix crashes on incorectly detected recursive aliases#18625
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
JukkaL
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Looks good. The logic is complicated, but users rely on this functionality so it's good that we support it.
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Feb 24, 2025
Fixes python#18505 Fixes python#16757 Fixing the crash is trivial, we simply give an error on something like `type X = X`, instead of crashing. But then I looked at what people actually want to do, they want to create an alias to something currently in scope (not a meaningless no-op alias). Right now we special-case classes/aliases to allow forward references (including recursive type aliases). However, I don't think we have any clear "scoping rules" for forward references. For example: ```python class C: Y = X class X: ... class X: ... ``` where `Y` should point to, `__main__.X` or `__main__.C.X`? Moreover, before this PR forward references can take precedence over real references: ```python class X: ... class C: Y = X # this resolves to __main__.C.X class X: ... ``` After some thinking I found this is not something I can fix in a simple PR. So instead I do just two things here: * Fix the actual crashes (and other potential similar crashes). * Add minimal change to accommodate the typical use case.
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Fixes #18505
Fixes #16757
Fixing the crash is trivial, we simply give an error on something like
type X = X, instead of crashing. But then I looked at what people actually want to do, they want to create an alias to something currently in scope (not a meaningless no-op alias). Right now we special-case classes/aliases to allow forward references (including recursive type aliases). However, I don't think we have any clear "scoping rules" for forward references. For example:where
Yshould point to,__main__.Xor__main__.C.X? Moreover, before this PR forward references can take precedence over real references:After some thinking I found this is not something I can fix in a simple PR. So instead I do just two things here: