From 6f957475ef1e84b04d6a022b9a0ce740d96d96c9 Mon Sep 17 00:00:00 2001
From: Douglas Creager <dcreager@dcreager.net>
Date: Tue, 5 Aug 2025 14:45:57 -0400
Subject: [PATCH] distributed union/intersection

---
 .../src/types/unification.rs                  | 24 +++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/crates/ty_python_semantic/src/types/unification.rs b/crates/ty_python_semantic/src/types/unification.rs
index 9a86072bbd..3e10cf8016 100644
--- a/crates/ty_python_semantic/src/types/unification.rs
+++ b/crates/ty_python_semantic/src/types/unification.rs
@@ -155,7 +155,7 @@ impl<'db> ConstraintSetSet<'db> {
         result
     }
 
-    /// union two sets of constraint sets.
+    /// Union two sets of constraint sets.
     ///
     /// This is the ⊔ operator from [[POPL2015][]], Definition 3.5.
     ///
@@ -165,6 +165,21 @@ impl<'db> ConstraintSetSet<'db> {
             self.add(db, set);
         }
     }
+
+    /// Calculate the distributed intersection of an iterator of sets of constraint sets.
+    fn distributed_intersection(db: &'db dyn Db, sets: impl IntoIterator<Item = Self>) -> Self {
+        sets.into_iter()
+            .fold(Self::always(), |sets, element| element.intersect(db, &sets))
+    }
+
+    /// Calculate the distributed union of an iterator of sets of constraint sets.
+    fn distributed_union(db: &'db dyn Db, sets: impl IntoIterator<Item = Self>) -> Self {
+        let mut result = Self::never();
+        for set in sets {
+            result.union(db, set);
+        }
+        result
+    }
 }
 
 impl<'db> From<Constraint<'db>> for ConstraintSetSet<'db> {
@@ -225,9 +240,10 @@ fn normalized_constraints_from_type_inner<'db>(
         Type::Union(union) => {
             // Figure 3, step 6
             // A union is a subtype of Never only if every element is.
-            (union.iter(db)).fold(ConstraintSetSet::always(), |sets, element| {
-                normalized_constraints_from_type(db, *element).intersect(db, &sets)
-            })
+            ConstraintSetSet::distributed_union(
+                db,
+                (union.iter(db)).map(|element| normalized_constraints_from_type(db, *element)),
+            )
         }
 
         Type::Intersection(intersection) => {