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Fix instance vs callable subtyping/assignability (#18260)

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## Summary

Fix some issues with subtying/assignability for instances vs callables.
We need to look up dunders on the class, not the instance, and we should
limit our logic here to delegating to the type of `__call__`, so it
doesn't get out of sync with the calls we allow.

Also, we were just entirely missing assignability handling for
`__call__` implemented as anything other than a normal bound method
(though we had it for subtyping.)

A first step towards considering what else we want to change in
https://github.com/astral-sh/ty/issues/491

## Test Plan

mdtests

---------

Co-authored-by: med <medioqrity@gmail.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
This commit is contained in:
Carl Meyer 2025-05-22 14:47:05 -05:00 • committed by GitHub
parent 0397682f1f
commit 0b181bc2ad
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4 changed files with 109 additions and 12 deletions
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41
crates/ty_python_semantic/src/types.rs
View file

@ -1291,12 +1291,20 @@ impl<'db> Type<'db> {
}
(Type::NominalInstance(_) | Type::ProtocolInstance(_), Type::Callable(_)) => {
let call_symbol = self.member(db, "__call__").symbol;
match call_symbol {
Symbol::Type(Type::BoundMethod(call_function), _) => call_function
.into_callable_type(db)
.is_subtype_of(db, target),
_ => false,
let call_symbol = self
.member_lookup_with_policy(
db,
Name::new_static("__call__"),
MemberLookupPolicy::NO_INSTANCE_FALLBACK,
)
.symbol;
// If the type of __call__ is a subtype of a callable type, this instance is.
// Don't add other special cases here; our subtyping of a callable type
// shouldn't get out of sync with the calls we will actually allow.
if let Symbol::Type(t, Boundness::Bound) = call_symbol {
t.is_subtype_of(db, target)
} else {
false
}
}
(Type::ProtocolInstance(left), Type::ProtocolInstance(right)) => {
@ -1641,12 +1649,20 @@ impl<'db> Type<'db> {
}
(Type::NominalInstance(_) | Type::ProtocolInstance(_), Type::Callable(_)) => {
let call_symbol = self.member(db, "__call__").symbol;
match call_symbol {
Symbol::Type(Type::BoundMethod(call_function), _) => call_function
.into_callable_type(db)
.is_assignable_to(db, target),
_ => false,
let call_symbol = self
.member_lookup_with_policy(
db,
Name::new_static("__call__"),
MemberLookupPolicy::NO_INSTANCE_FALLBACK,
)
.symbol;
// If the type of __call__ is assignable to a callable type, this instance is.
// Don't add other special cases here; our assignability to a callable type
// shouldn't get out of sync with the calls we will actually allow.
if let Symbol::Type(t, Boundness::Bound) = call_symbol {
t.is_assignable_to(db, target)
} else {
false
}
}
@ -2746,6 +2762,7 @@ impl<'db> Type<'db> {
instance.display(db),
owner.display(db)
);
let descr_get = self.class_member(db, "__get__".into()).symbol;
if let Symbol::Type(descr_get, descr_get_boundness) = descr_get {