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assert_never not being properly handled when using TypeVars bound to Union Type #15467

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ELC opened this issue Jun 19, 2023 · 2 comments
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assert_never not being properly handled when using TypeVars bound to Union Type #15467

ELC opened this issue Jun 19, 2023 · 2 comments
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bug mypy got something wrong topic-type-narrowing Conditional type narrowing / binder topic-type-variables

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@ELC
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ELC commented Jun 19, 2023

Bug Report

When using exhaustive checking either in if/elif/else or with match over TypeVars bound to Union Types mypy throws:

Argument 1 to "assert_never" has incompatible type "T"; expected "NoReturn"  [arg-type]
expected "NoReturn"  [arg-type]

Whereas this should have been the expected behavior and no errors should be raised.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=c90f6c776bc8d64645075dee9bf15032

Expected Behavior

The expected behavior should be identical as if operating over the Union Type, not throwing erros as the type has been exhausted,

Actual Behavior

An error is shown:

Argument 1 to "assert_never" has incompatible type "T"; expected "NoReturn"  [arg-type]
expected "NoReturn"  [arg-type]

Your Environment

  • Mypy version used: 1.3.0
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.11
@ELC ELC added the bug mypy got something wrong label Jun 19, 2023
@AlexWaygood AlexWaygood added topic-type-variables topic-type-narrowing Conditional type narrowing / binder labels Jun 19, 2023
@kopp kopp mentioned this issue Jun 20, 2023
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@slanzmich
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This issue seems to be fixed in mypy 1.12.0.

@brianschubert
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Thanks! Duplicate of #15235, this was fixed in #17850

@brianschubert brianschubert closed this as not planned Won't fix, can't repro, duplicate, stale Nov 19, 2024
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@ELC @brianschubert @slanzmich @AlexWaygood