do not remove __typename from input values

NEXT_CHANGELOG.md

@@ -79,8 +79,11 @@ Spans generated by the Router are now aligned with plugin services.
 All windows processes are spawned via xtask rather than a separate CircleCI stage.
 
 ### Enable default feature in graphql_client [PR #905](https://github.com/apollographql/router/pull/905)
-removing the default feature can cause build issues in plugins.
+Removing the default feature can cause build issues in plugins.
 
-## 📚 Documentation
+### Do not remove __typename from the aggregated response [PR #919](https://github.com/apollographql/router/pull/919)
+If the client was explicitely requesting the `__typename` field, it was removed from the aggregated subgraph data, and so was not usable by fragment to check the type.
+
+ ## 📚 Documentation
 ### Enhanced rust docs ([PR #819](https://github.com/apollographql/router/pull/819))
 Many more rust docs have been added.

apollo-router-core/src/spec/query.rs

@@ -338,17 +338,24 @@ impl Query {
                         if input_value.is_null() && output.contains_key(field_name.as_str()) {
                             continue;
                         }
+
                         let selection_set = selection_set.as_deref().unwrap_or_default();
                         let output_value =
                             output.entry((*field_name).clone()).or_insert(Value::Null);
-                        self.format_value(
-                            field_type,
-                            variables,
-                            input_value,
-                            output_value,
-                            selection_set,
-                            schema,
-                        )?;
+                        if field_name.as_str() == "__typename" {
+                            if input_value.is_string() {
+                                *output_value = input_value.clone();
+                            }
+                        } else {
+                            self.format_value(
+                                field_type,
+                                variables,
+                                input_value,
+                                output_value,
+                                selection_set,
+                                schema,
+                            )?;
+                        }
                     } else {
                         if !output.contains_key(field_name.as_str()) {
                             output.insert((*field_name).clone(), Value::Null);
@@ -3716,4 +3723,48 @@ mod tests {
             }},
         );
     }
+
+    #[test]
+    fn union_with_typename() {
+        let schema = "type Query {
+            get: ProductResult
+        }
+
+        type Product{
+            symbol: String!
+        }
+        type ProductError{
+            reason: String
+        }
+        union ProductResult = Product | ProductError
+        ";
+
+        assert_format_response!(
+            schema,
+            "query  {
+                get {
+                    __typename
+                    ... on Product {
+                      symbol
+                    }
+                    ... on ProductError {
+                      reason
+                    }
+                }
+            }",
+            json! {{
+                "get": {
+                    "__typename": "Product",
+                    "symbol": "1"
+                },
+            }},
+            None,
+            json! {{
+                "get": {
+                    "__typename": "Product",
+                    "symbol": "1"
+                },
+            }},
+        );
+    }
 }