[red-knot] Add definitions and limited type inference for exception handlers (#13267)

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -880,6 +880,30 @@ where
 
         self.current_match_case.as_mut().unwrap().index += 1;
     }
+
+    fn visit_except_handler(&mut self, except_handler: &'ast ast::ExceptHandler) {
+        let ast::ExceptHandler::ExceptHandler(except_handler) = except_handler;
+        let ast::ExceptHandlerExceptHandler {
+            name: symbol_name,
+            type_: handled_exceptions,
+            body,
+            range: _,
+        } = except_handler;
+
+        if let Some(handled_exceptions) = handled_exceptions {
+            self.visit_expr(handled_exceptions);
+        }
+
+        // If `handled_exceptions` above was `None`, it's something like `except as e:`,
+        // which is invalid syntax. However, it's still pretty obvious here that the user
+        // *wanted* `e` to be bound, so we should still create a definition here nonetheless.
+        if let Some(symbol_name) = symbol_name {
+            let symbol = self.add_or_update_symbol(symbol_name.id.clone(), SymbolFlags::IS_DEFINED);
+            self.add_definition(symbol, except_handler);
+        }
+
+        self.visit_body(body);
+    }
 }
 
 #[derive(Copy, Clone, Debug)]