fix fancy slicing in Nx

CHANGES.md

@@ -30,6 +30,7 @@ We're closing 8 user-reported issues or feature requests and are totalling 30 co
 
 ### Nx
 
+- Fix fancy slicing so `L [...]` with full axis length correctly supports permutations and duplicates; only identity order is a no‑op (#152, @Arsalaan-Alam)
 - Fix einsum output axis ordering for free axes (e.g., `i,jk->jki`, `ij,klj->kli`) by correcting final transpose permutation and intermediate left-axis reordering. (@tmattio)
 - Add `Nx_io.Cache_dir` module with consolidated cache directory utilities respecting `RAVEN_CACHE_ROOT`, `XDG_CACHE_HOME`, and `HOME` fallback, replacing project-specific cache logic across the whole raven ecosystem (#134, @Arsalaan-Alam)
 - Add `Nx_io.save_txt` / `Nx_io.load_txt` with NumPy-compatible formatting, comments, and dtype support (#120, @six-shot)

nx/lib/core/frontend.ml

@@ -2969,7 +2969,18 @@ module Make (B : Backend_intf.S) = struct
                           let dim_size = (shape tensor).(working_dim) in
                           let indices = indices_of_spec dim_size spec in
                           let tensor' =
-                            if List.length indices = dim_size then tensor
+                            (* If indices cover the whole dimension in identity
+                               order, this is a no-op. Otherwise we must gather
+                               (to support permutations and duplicates). *)
+                            let is_range n indices =
+                              let rec traverse id = function
+                                | [] -> id = n
+                                | hd :: tl when hd = id -> traverse (succ id) tl
+                                | _ -> false
+                              in
+                              traverse 0 indices
+                            in
+                            if is_range dim_size indices then tensor
                             else if List.length indices = 0 then (
                               (* Empty slice - create tensor with 0 size in this
                                  dimension *)

nx/test/test_nx_indexing.ml

@@ -78,6 +78,18 @@ let test_index_mixed () =
     [| 20.; 21.; 22.; 23.; 24.; 30.; 31.; 32.; 33.; 34. |]
     indexed
 
+(* Fancy indexing along axis 0 should reorder rows (and support duplicates) *)
+let test_index_idx_reorder_rows () =
+  let t = Nx.create Nx.float32 [| 3; 2 |] (Array.init 6 float_of_int) in
+  let indexed = Nx.slice [ Nx.L [ 2; 0; 1 ]; Nx.A ] t in
+  check_t "index idx reorder rows" [| 3; 2 |] [| 4.; 5.; 0.; 1.; 2.; 3. |] indexed
+
+let test_index_idx_duplicate_rows () =
+  let t = Nx.create Nx.float32 [| 3; 2 |] (Array.init 6 float_of_int) in
+  let indexed = Nx.slice [ Nx.L [ 1; 1; 0 ]; Nx.A ] t in
+  check_t "index idx duplicate rows" [| 3; 2 |] [| 2.; 3.; 2.; 3.; 0.; 1. |]
+    indexed
+
 (* Note: `new_ and `mask require implementation *)
 (* let test_index_new_axis  () =
     let t = Nx.create  Nx.float32 [| 3; 4 |] (Array.init 12 float_of_int) in
@@ -396,6 +408,8 @@ let index_tests =
     ("index idx", `Quick, test_index_idx);
     ("index idx repeated", `Quick, test_index_idx_repeated);
     ("index mixed", `Quick, test_index_mixed);
+    ("index idx reorder rows", `Quick, test_index_idx_reorder_rows);
+    ("index idx duplicate rows", `Quick, test_index_idx_duplicate_rows);
     ("set_slice at", `Quick, test_set_slice_at);
     ("set_slice rng", `Quick, test_set_slice_rng);
     ("set_slice idx", `Quick, test_set_slice_idx);