fix: correct comparison of variable values (#54)

Author: vstepanyuk

src/ast/ext.rs

@@ -325,6 +325,7 @@ impl ValueExtension for Value {
             (Value::Object(a), Value::Object(b)) => {
                 a.iter().zip(b.iter()).all(|(a, b)| a.1.compare(b.1))
             }
+            (Value::Variable(a), Value::Variable(b)) => a.eq(b),
             _ => false,
         }
     }

src/validation/rules/overlapping_fields_can_be_merged.rs

@@ -941,6 +941,42 @@ fn identical_fields() {
     assert_eq!(get_messages(&errors).len(), 0);
 }
 
+#[test]
+fn identical_fields_with_identical_variables() {
+    use crate::validation::test_utils::*;
+
+    let mut plan = create_plan_from_rule(Box::new(OverlappingFieldsCanBeMerged::new()));
+    let errors = test_operation_with_schema(
+        r#"fragment mergeIdenticalFieldsWithIdenticalArgs on Dog {
+          doesKnowCommand(dogCommand: $dogCommand)
+          doesKnowCommand(dogCommand: $dogCommand)
+        }"#,
+        TEST_SCHEMA,
+        &mut plan,
+    );
+
+    assert_eq!(get_messages(&errors).len(), 0);
+}
+
+#[test]
+fn identical_fields_with_different_variables() {
+    use crate::validation::test_utils::*;
+
+    let mut plan = create_plan_from_rule(Box::new(OverlappingFieldsCanBeMerged::new()));
+    let errors = test_operation_with_schema(
+        r#"fragment mergeIdenticalFieldsWithIdenticalArgs on Dog {
+          doesKnowCommand(dogCommand: $catCommand)
+          doesKnowCommand(dogCommand: $dogCommand)
+        }"#,
+        TEST_SCHEMA,
+        &mut plan,
+    );
+
+    let messages = get_messages(&errors);
+    assert_eq!(messages.len(), 1);
+    assert_eq!(messages, vec!["Fields \"doesKnowCommand\" conflict because they have differing arguments. Use different aliases on the fields to fetch both if this was intentional."]);
+}
+
 #[test]
 fn identical_fields_and_identical_args() {
     use crate::validation::test_utils::*;