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Avoid erasing type objects when checking runtime cover #19320

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Merged
merged 5 commits into from
Jun 22, 2025
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Avoid erasing type objects when checking runtime cover #19320

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08a9a22
Only erase instances when checking runtime cover
hauntsaninja Jun 20, 2025
5974152
.
hauntsaninja Jun 20, 2025
c7452b5
Merge branch 'master' into coveruntime
hauntsaninja Jun 21, 2025
aab250c
test
hauntsaninja Jun 22, 2025
d477fe7
erase
hauntsaninja Jun 22, 2025
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3 changes: 2 additions & 1 deletion mypy/subtypes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2106,7 +2106,8 @@ def covers_at_runtime(item: Type, supertype: Type) -> bool:
supertype = get_proper_type(supertype)

# Since runtime type checks will ignore type arguments, erase the types.
supertype = erase_type(supertype)
if not isinstance(supertype, CallableType):
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@JukkaL JukkaL Jun 21, 2025

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Should we only skip erase if the callable type represents a type object? What about overloaded callable types?

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@hauntsaninja hauntsaninja Jun 22, 2025

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I'm not sure how we would get here with other CallableType's (and we don't anywhere in the test suite). isinstance(item, CallableType) isn't really a thing that can happen at runtime.

I went ahead and restricted it to just type objects though!

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@ilevkivskyi ilevkivskyi Jun 25, 2025

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IIUC it can be triggered (at least in theory) by if callable(x): ... checks. Also as @JukkaL said you should really use FunctionLike instead of CallableType, otherwise things will weirdly behave differently for classes with overloaded constructors (unfortunately this already happens in a bunch of other places, but it is not an excuse to add more of it).

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@hauntsaninja hauntsaninja Jun 25, 2025

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Oh lol right, I even made an issue about this a few years ago #12320 (and tried to fix it in several places)

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@hauntsaninja hauntsaninja Jun 25, 2025

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#19338

supertype = erase_type(supertype)
if is_proper_subtype(
erase_type(item), supertype, ignore_promotions=True, erase_instances=True
):
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21 changes: 21 additions & 0 deletions test-data/unit/check-python310.test
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Expand Up @@ -2601,6 +2601,27 @@ def f(t: T) -> None:
reveal_type(k) # N: Revealed type is "tuple[builtins.int, fallback=__main__.K]"
[builtins fixtures/tuple.pyi]

[case testMatchTypeObjectTypeVar]
from typing import TypeVar
import b

T_Choice = TypeVar("T_Choice", bound=b.One | b.Two)

def switch(choice: type[T_Choice]) -> None:
match choice:
case b.One:
reveal_type(choice) # N: Revealed type is "def () -> b.One"
case b.Two:
reveal_type(choice) # N: Revealed type is "def () -> b.Two"
case _:
reveal_type(choice) # N: Revealed type is "type[T_Choice`-1]"

[file b.py]
class One: ...
class Two: ...

[builtins fixtures/tuple.pyi]

[case testNewRedefineMatchBasics]
# flags: --allow-redefinition-new --local-partial-types

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