merge: correctly identify conflicts

Fix a bug and correctly identify merge conflicts between inserts and
deletes.

src/merge/mod.rs

@@ -453,10 +453,14 @@ fn create_merge_range<'ancestor, 'ours, 'theirs, T: ?Sized + SliceLike>(
         (None, Some(ours), None) => Some(MergeRange::Ours(ours)),
         (None, None, Some(theirs)) => Some(MergeRange::Theirs(theirs)),
 
-        (Some(_), Some(_), None)
-        | (Some(_), None, Some(_))
-        | (Some(_), None, None)
-        | (None, None, None) => None,
+        (Some(ancestor), None, Some(theirs)) => {
+            Some(MergeRange::Conflict(ancestor, Range::empty(), theirs))
+        }
+        (Some(ancestor), Some(ours), None) => {
+            Some(MergeRange::Conflict(ancestor, ours, Range::empty()))
+        }
+
+        (Some(_), None, None) | (None, None, None) => None,
     }
 }
 

src/merge/tests.rs

@@ -343,3 +343,64 @@ class Call(models.Model):
 
     assert_merge!(base, ours, theirs, Ok(expected), "MergeRange::Both case");
 }
+
+#[test]
+fn delete_and_insert_conflict() {
+    let base = r#"
+{
+    int a = 2;
+}
+"#;
+
+    let ours = r#"
+{
+}
+"#;
+
+    let theirs = r#"
+{
+    int a = 2;
+    int b = 3;
+}
+"#;
+
+    let expected = r#"
+{
+<<<<<<< ours
+||||||| original
+    int a = 2;
+=======
+    int a = 2;
+    int b = 3;
+>>>>>>> theirs
+}
+"#;
+
+    assert_merge!(
+        base,
+        ours,
+        theirs,
+        Err(expected),
+        "MergeRange (Ours::delete, Theirs::insert) conflict"
+    );
+
+    let expected = r#"
+{
+<<<<<<< ours
+    int a = 2;
+    int b = 3;
+||||||| original
+    int a = 2;
+=======
+>>>>>>> theirs
+}
+"#;
+
+    assert_merge!(
+        base,
+        theirs,
+        ours,
+        Err(expected),
+        "MergeRange (Theirs::delete, Ours::insert) conflict"
+    );
+}