[ty] Propagate type context through conditional expressions (#21443)

## Summary

Resolves https://github.com/astral-sh/ty/issues/1543.

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -341,8 +341,6 @@ d: X[Any] = X(1)
 reveal_type(d)  # revealed: X[Any]
 
 def _(flag: bool):
-    # TODO: Handle unions correctly.
-    # error: [invalid-assignment] "Object of type `X[int]` is not assignable to `X[int | None]`"
     a: X[int | None] = X(1) if flag else X(2)
     reveal_type(a)  # revealed: X[int | None]
 ```

crates/ty_python_semantic/resources/mdtest/bidirectional.md

@@ -281,6 +281,27 @@ A(f(1))
 A(f([]))
 ```
 
+## Conditional expressions
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+The type context is propagated through both branches of conditional expressions:
+
+```py
+def f[T](x: T) -> list[T]:
+    raise NotImplementedError
+
+def _(flag: bool):
+    x1 = f(1) if flag else f(2)
+    reveal_type(x1)  # revealed: list[Literal[1]] | list[Literal[2]]
+
+    x2: list[int | None] = f(1) if flag else f(2)
+    reveal_type(x2)  # revealed: list[int | None]
+```
+
 ## Multi-inference diagnostics
 
 ```toml

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -6894,7 +6894,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             ast::Expr::Compare(compare) => self.infer_compare_expression(compare),
             ast::Expr::Subscript(subscript) => self.infer_subscript_expression(subscript),
             ast::Expr::Slice(slice) => self.infer_slice_expression(slice),
-            ast::Expr::If(if_expression) => self.infer_if_expression(if_expression),
+            ast::Expr::If(if_expression) => self.infer_if_expression(if_expression, tcx),
             ast::Expr::Lambda(lambda_expression) => self.infer_lambda_expression(lambda_expression),
             ast::Expr::Call(call_expression) => self.infer_call_expression(call_expression, tcx),
             ast::Expr::Starred(starred) => self.infer_starred_expression(starred),
@@ -7740,7 +7740,11 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         })
     }
 
-    fn infer_if_expression(&mut self, if_expression: &ast::ExprIf) -> Type<'db> {
+    fn infer_if_expression(
+        &mut self,
+        if_expression: &ast::ExprIf,
+        tcx: TypeContext<'db>,
+    ) -> Type<'db> {
         let ast::ExprIf {
             range: _,
             node_index: _,
@@ -7750,8 +7754,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         } = if_expression;
 
         let test_ty = self.infer_standalone_expression(test, TypeContext::default());
-        let body_ty = self.infer_expression(body, TypeContext::default());
+        let body_ty = self.infer_expression(body, tcx);
-        let orelse_ty = self.infer_expression(orelse, TypeContext::default());
+        let orelse_ty = self.infer_expression(orelse, tcx);
 
         match test_ty.try_bool(self.db()).unwrap_or_else(|err| {
             err.report_diagnostic(&self.context, &**test);

crates/ty_python_semantic/src/types/infer/builder/type_expression.rs

@@ -330,7 +330,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
             }
 
             ast::Expr::If(if_expression) => {
-                self.infer_if_expression(if_expression);
+                self.infer_if_expression(if_expression, TypeContext::default());
                 self.report_invalid_type_expression(
                     expression,
                     format_args!("`if` expressions are not allowed in type expressions"),