fix: catch missing fragment definitions in standalone executable validation (#1003)

Given an operation with undefined fragment definitions using `document.validate_standalone_executable()`, report `UndefinedFragment` error.

Author: Abdel-Monaam-Aouini

crates/apollo-compiler/src/validation/field.rs

@@ -33,10 +33,18 @@ pub(crate) fn validate_field(
 
     super::argument::validate_arguments(diagnostics, &field.arguments);
 
-    // Return early if we don't know the type--this can happen if we are nested deeply
-    // inside a selection set that has a wrong field, or if we are validating a standalone
-    // operation without a schema.
+    // If we don't know the type (no schema, or invalid parent), we cannot perform
+    // type-aware checks for this field. However, for standalone executable validation
+    // we still want to traverse into the nested selection set so that validations
+    // that do not require a schema (like missing fragment detection) can run.
     let Some((schema, against_type)) = against_type else {
+        super::selection::validate_selection_set(
+            diagnostics,
+            document,
+            None,
+            &field.selection_set,
+            context,
+        );
         return;
     };
 

crates/apollo-compiler/tests/validation/mod.rs

@@ -352,3 +352,33 @@ type TestObject {
         }"#]];
     expected.assert_eq(&actual);
 }
+
+#[test]
+fn missing_fragment_in_standalone_validation() {
+    let input = r#"
+        query {
+            company {
+                user {
+                  ...UserFragment
+                }
+
+                ...CompanyFragment
+            }
+        }
+
+        fragment UserFragment on User {
+            id
+            name
+        }
+    "#;
+
+    let doc = ast::Document::parse(input, "query.graphql").unwrap();
+    let diagnostics = doc
+        .validate_standalone_executable()
+        .expect_err("should report missing fragment error");
+    let errors = diagnostics.to_string();
+    assert!(
+        errors.contains("cannot find fragment `CompanyFragment` in this document"),
+        "{errors}"
+    );
+}