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Fix stack overflow during subscript inference on recursive type aliases #4635

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Fix stack overflow during subscript inference on recursive type aliases #4635

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66 changes: 53 additions & 13 deletions pyrefly/lib/alt/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,7 @@ use pyrefly_types::shaped_array::index_shape_tensor;
use pyrefly_types::shaped_array::shape_to_tuple_carrier;
use pyrefly_types::shaped_array::tuple_carrier_to_shape;
use pyrefly_types::shaped_array::type_to_dim;
use pyrefly_types::type_alias::TypeAliasData;
use pyrefly_types::type_level_dsl::TypeShapeDslDomain;
use pyrefly_types::typed_dict::AnonymousTypedDictInner;
use pyrefly_types::typed_dict::ExtraItems;
Expand Down Expand Up @@ -3133,6 +3134,28 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
errors: &ErrorCollector,
key_present: bool, // true if the key is definitely known to be present
type_form_context: TypeFormContext<'_>,
) -> Type {
let mut aliases = SmallSet::new();
self.subscript_infer_for_type_with_key_present_inner(
base,
slice,
range,
errors,
key_present,
type_form_context,
&mut aliases,
)
}

fn subscript_infer_for_type_with_key_present_inner(
&self,
base: &Type,
slice: &Expr,
range: TextRange,
errors: &ErrorCollector,
key_present: bool, // true if the key is definitely known to be present
type_form_context: TypeFormContext<'_>,
aliases: &mut SmallSet<TypeAliasData>,
) -> Type {
let xs = Ast::unpack_slice(slice);
let slice_ty = LazyCell::new(|| self.expr_infer(slice, errors));
Expand Down Expand Up @@ -3510,13 +3533,14 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
}
}
}
let result = self.subscript_infer_for_type_with_key_present(
let result = self.subscript_infer_for_type_with_key_present_inner(
&schema.underlying_type(),
slice,
range,
errors,
key_present,
type_form_context,
aliases,
);
// Preserve the stub's Series class when attaching an element dtype.
match (column_dtype, result) {
Expand All @@ -3532,45 +3556,49 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
(_, result) => result,
}
}
Type::Series(schema) => self.subscript_infer_for_type_with_key_present(
Type::Series(schema) => self.subscript_infer_for_type_with_key_present_inner(
&schema.underlying_type(),
slice,
range,
errors,
key_present,
type_form_context,
aliases,
),
Type::Quantified(ref q) if q.is_type_var() && q.restriction().is_restricted() => {
match q.restriction() {
Restriction::Bound(bound) => self
.subscript_infer_for_type_with_key_present(
.subscript_infer_for_type_with_key_present_inner(
bound,
slice,
range,
errors,
key_present,
type_form_context,
aliases,
),
Restriction::Constraints(constraints) => {
self.unions(constraints.map(|constraint| {
self.subscript_infer_for_type_with_key_present(
self.subscript_infer_for_type_with_key_present_inner(
constraint,
slice,
range,
errors,
key_present,
type_form_context,
aliases,
)
}))
}
Restriction::Flag(domain) => self
.subscript_infer_for_type_with_key_present(
.subscript_infer_for_type_with_key_present_inner(
&domain.as_type(self.stdlib, self.heap),
slice,
range,
errors,
key_present,
type_form_context,
aliases,
),
Restriction::Unrestricted => {
unreachable!("restricted TypeVar cannot be unrestricted")
Expand Down Expand Up @@ -3657,14 +3685,26 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
}
})
}
Type::UntypedAlias(ta) => self.subscript_infer_for_type_with_key_present(
&self.untype_alias(&ta),
slice,
range,
errors,
key_present,
type_form_context,
),
Type::UntypedAlias(ta) => {
// Recursive aliases can contain a mapping whose value is the alias itself.
// A subscript on that value cannot be made more precise without expanding
// the alias again, so stop the cycle with an implicit Any.
if !aliases.insert((*ta).clone()) {
self.heap.mk_any_implicit()
} else {
let result = self.subscript_infer_for_type_with_key_present_inner(
&self.untype_alias(&ta),
slice,
range,
errors,
key_present,
type_form_context,
aliases,
);
aliases.shift_remove(&*ta);
result
}
}
t => self.error(
errors,
range,
Expand Down
16 changes: 16 additions & 0 deletions pyrefly/lib/test/pattern_match.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -59,6 +59,22 @@ match None: # E: Missing cases: None
"#,
);

// Regression test for https://github.com/facebook/pyrefly/issues/4631.
testcase!(
test_recursive_alias_mapping_pattern_does_not_overflow,
r#"
from typing import Mapping

def fn(obj: U):
match obj:
case {'a': {'b': []}}:
pass

T = 'T' | str | Mapping[str, 'T'] # E: `|` union syntax does not work with string literals # E: Found cyclic self-reference in `T`
U = Mapping[str, T]
"#,
);

testcase!(
test_match_case_unreachable_for_disjoint_subject_type,
r#"
Expand Down
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