Respect intersections in iterations

crates/ty_python_semantic/resources/mdtest/loops/for.md

@@ -337,6 +337,31 @@ for x in Test():
     reveal_type(x)  # revealed: int
 ```
 
+## Intersection type via isinstance narrowing
+
+When we have an intersection type via `isinstance` narrowing, we should be able to infer the
+iterable element type precisely:
+
+```py
+from typing import Sequence
+
+def _(x: Sequence[int], y: object):
+    reveal_type(x)  # revealed: Sequence[int]
+    for item in x:
+        reveal_type(item)  # revealed: int
+
+    if isinstance(y, list):
+        reveal_type(y)  # revealed: Top[list[Unknown]]
+        for item in y:
+            reveal_type(item)  # revealed: object
+
+    if isinstance(x, list):
+        reveal_type(x)  # revealed: Sequence[int] & Top[list[Unknown]]
+        for item in x:
+            # int & object simplifies to int
+            reveal_type(item)  # revealed: int
+```
+
 ## Possibly-not-callable `__iter__` method
 
 ```py

crates/ty_python_semantic/src/types.rs

@@ -6612,6 +6612,17 @@ impl<'db> Type<'db> {
                         None
                     }
                 }
+                Type::Intersection(intersection) => {
+                    // For intersections, we iterate over each positive element and intersect
+                    // the resulting element types. Negative elements don't affect iteration.
+                    let mut elements_iter = intersection.positive_elements_or_object(db);
+                    let first_element_spec = elements_iter.next()?.try_iterate_with_mode(db, EvaluationMode::Sync).ok()?;
+                    let mut builder = TupleSpecBuilder::from(&*first_element_spec);
+                    for element in elements_iter {
+                        builder = builder.intersect(db, &*element.try_iterate_with_mode(db, EvaluationMode::Sync).ok()?);
+                    }
+                    Some(Cow::Owned(builder.build()))
+                }
                 // N.B. These special cases aren't strictly necessary, they're just obvious optimizations
                 Type::LiteralString | Type::Dynamic(_) => Some(Cow::Owned(TupleSpec::homogeneous(ty))),
 
@@ -6634,7 +6645,6 @@ impl<'db> Type<'db> {
                 | Type::SpecialForm(_)
                 | Type::KnownInstance(_)
                 | Type::PropertyInstance(_)
-                | Type::Intersection(_)
                 | Type::AlwaysTruthy
                 | Type::AlwaysFalsy
                 | Type::IntLiteral(_)

crates/ty_python_semantic/src/types/tuple.rs

@@ -29,8 +29,8 @@ use crate::types::constraints::{ConstraintSet, IteratorConstraintsExtension};
 use crate::types::generics::InferableTypeVars;
 use crate::types::{
     ApplyTypeMappingVisitor, BoundTypeVarInstance, FindLegacyTypeVarsVisitor, HasRelationToVisitor,
-    IsDisjointVisitor, IsEquivalentVisitor, NormalizedVisitor, Type, TypeMapping, TypeRelation,
-    UnionBuilder, UnionType,
+    IntersectionType, IsDisjointVisitor, IsEquivalentVisitor, NormalizedVisitor, Type, TypeMapping,
+    TypeRelation, UnionBuilder, UnionType,
 };
 use crate::types::{Truthiness, TypeContext};
 use crate::{Db, FxOrderSet, Program};
@@ -1787,6 +1787,42 @@ impl<'db> TupleSpecBuilder<'db> {
         }
     }
 
+    /// Return a new tuple-spec builder that reflects the intersection of this tuple and another tuple.
+    ///
+    /// For example, if `self` is a tuple-spec builder for `tuple[int, str]` and `other` is a
+    /// tuple-spec for `tuple[object, object]`, the result will be a tuple-spec builder for
+    /// `tuple[int, str]` (since `int & object` simplifies to `int`, and `str & object` to `str`).
+    ///
+    /// To keep things simple, we currently only attempt to preserve the "fixed-length-ness" of
+    /// a tuple spec if both `self` and `other` have the exact same length. For example,
+    /// if `self` is a tuple-spec builder for `tuple[int, str]` and `other` is a tuple-spec for
+    /// `tuple[int, str, bytes]`, the result will be a tuple-spec builder for
+    /// `tuple[int & str & bytes, ...]`.
+    pub(crate) fn intersect(mut self, db: &'db dyn Db, other: &TupleSpec<'db>) -> Self {
+        match (&mut self, other) {
+            (TupleSpecBuilder::Fixed(our_elements), TupleSpec::Fixed(new_elements))
+                if our_elements.len() == new_elements.len() =>
+            {
+                for (existing, new) in our_elements.iter_mut().zip(new_elements.elements()) {
+                    *existing = IntersectionType::from_elements(db, [*existing, *new]);
+                }
+                self
+            }
+
+            _ => {
+                let intersected = IntersectionType::from_elements(
+                    db,
+                    self.all_elements().chain(other.all_elements()),
+                );
+                TupleSpecBuilder::Variable {
+                    prefix: vec![],
+                    variable: intersected,
+                    suffix: vec![],
+                }
+            }
+        }
+    }
+
     pub(super) fn build(self) -> TupleSpec<'db> {
         match self {
             TupleSpecBuilder::Fixed(elements) => {