[ty] Simplify an intersection of N & ~T to Never if B & ~T would simplify to Never, where B is the concrete base type of a NewType N (#24086)

AlexWaygood · web-flow · commit 06256af8e552 · 2026-03-20T19:14:30.000Z
## Summary Fixes astral-sh/ty#2941 Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com> ## Test Plan added an mdtest
diff --git a/crates/ty_python_semantic/resources/mdtest/annotations/new_types.md b/crates/ty_python_semantic/resources/mdtest/annotations/new_types.md
@@ -429,6 +429,17 @@ def _(x: Foo | float, y: Bar | complex):
     reveal_type(not y)  # revealed: bool
 ```
 
+Intersections with `NewType`s solve to `Never` if the intersection with the `NewType`'s concrete
+base type would also solve to `Never`:
+
+```py
+TFloat = NewType("TFloat", float)
+
+def f(x: TFloat) -> None:
+    if not isinstance(x, float | int):
+        reveal_type(x)  # revealed: Never
+```
+
 ## A `NewType` definition must be a simple variable assignment
 
 ```py
diff --git a/crates/ty_python_semantic/src/types/set_theoretic/builder.rs b/crates/ty_python_semantic/src/types/set_theoretic/builder.rs
@@ -1516,7 +1516,11 @@ impl<'db> InnerIntersectionBuilder<'db> {
         // to their upper bound and all constrained type variables to the union of their constraints.
         // If that speculative intersection simplifies to `Never`, this intersection must also simplify
         // to `Never`.
-        if self.positive.iter().any(|ty| ty.is_type_var()) {
+        if self
+            .positive
+            .iter()
+            .any(|ty| matches!(ty, Type::TypeVar(_) | Type::NewTypeInstance(_)))
+        {
             let mut speculative = IntersectionBuilder::new(db);
             for pos in &self.positive {
                 match pos {
@@ -1533,6 +1537,9 @@ impl<'db> InnerIntersectionBuilder<'db> {
                             None => {}
                         }
                     }
+                    Type::NewTypeInstance(newtype) => {
+                        speculative = speculative.add_positive(newtype.concrete_base_type(db));
+                    }
                     _ => speculative = speculative.add_positive(*pos),
                 }
             }

Original file line number	Diff line number	Diff line change
`@@ -1516,7 +1516,11 @@ impl<'db> InnerIntersectionBuilder<'db> {`
`1516`	`1516`	`// to their upper bound and all constrained type variables to the union of their constraints.`
`1517`	`1517`	// If that speculative intersection simplifies to `Never`, this intersection must also simplify
`1518`	`1518`	// to `Never`.
`1519`		`- if self.positive.iter().any(\|ty\| ty.is_type_var()) {`
	`1519`	`+ if self`
	`1520`	`+ .positive`
	`1521`	`+ .iter()`
	`1522`	`+ .any(\|ty\| matches!(ty, Type::TypeVar(_) \| Type::NewTypeInstance(_)))`
	`1523`	`+ {`
`1520`	`1524`	`let mut speculative = IntersectionBuilder::new(db);`
`1521`	`1525`	`for pos in &self.positive {`
`1522`	`1526`	`match pos {`
`@@ -1533,6 +1537,9 @@ impl<'db> InnerIntersectionBuilder<'db> {`
`1533`	`1537`	`None => {}`
`1534`	`1538`	`}`
`1535`	`1539`	`}`
	`1540`	`+ Type::NewTypeInstance(newtype) => {`
	`1541`	`+ speculative = speculative.add_positive(newtype.concrete_base_type(db));`
	`1542`	`+ }`
`1536`	`1543`	`_ => speculative = speculative.add_positive(*pos),`
`1537`	`1544`	`}`
`1538`	`1545`	`}`