From df1ddf3b30f2a5c80c118058027115dd00b847e0 Mon Sep 17 00:00:00 2001
From: Pablo Brubeck <brubeck@protonmail.com>
Date: Wed, 15 Jan 2025 12:32:37 +0000
Subject: [PATCH] Indexed: only initialise new instances

---
 test/test_simplify.py | 19 +++++++++++++++++++
 ufl/indexed.py        | 13 +++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/test/test_simplify.py b/test/test_simplify.py
index 9eff3843b..dcd3e06ba 100755
--- a/test/test_simplify.py
+++ b/test/test_simplify.py
@@ -32,7 +32,9 @@
     triangle,
 )
 from ufl.algorithms import compute_form_data
+from ufl.core.multiindex import FixedIndex, MultiIndex
 from ufl.finiteelement import FiniteElement
+from ufl.indexed import Indexed
 from ufl.pullback import identity_pullback
 from ufl.sobolevspace import H1
 
@@ -174,3 +176,20 @@ def test_tensor_from_indexed(self, shape):
     space = FunctionSpace(domain, element)
     f = Coefficient(space)
     assert as_tensor(reshape([f[i] for i in ndindex(f.ufl_shape)], f.ufl_shape).tolist()) is f
+
+
+def test_nested_indexed(self):
+    # Test that a nested Indexed expression simplifies to the existing Indexed object
+    shape = (2,)
+    element = FiniteElement("Lagrange", triangle, 1, shape, identity_pullback, H1)
+    domain = Mesh(FiniteElement("Lagrange", triangle, 1, (2,), identity_pullback, H1))
+    space = FunctionSpace(domain, element)
+    f = Coefficient(space)
+
+    comps = tuple(f[i] for i in range(2))
+    assert all(isinstance(c, Indexed) for c in comps)
+    expr = as_tensor(list(reversed(comps)))
+
+    multiindex = MultiIndex((FixedIndex(0),))
+    assert Indexed(expr, multiindex) is expr[0]
+    assert Indexed(expr, multiindex) is comps[1]
diff --git a/ufl/indexed.py b/ufl/indexed.py
index f4433f223..338033413 100644
--- a/ufl/indexed.py
+++ b/ufl/indexed.py
@@ -20,6 +20,7 @@ class Indexed(Operator):
     """Indexed expression."""
 
     __slots__ = (
+        "_initialised",
         "ufl_free_indices",
         "ufl_index_dimensions",
     )
@@ -46,13 +47,16 @@ def __new__(cls, expression, multiindex):
 
         try:
             # Simplify indexed ListTensor
-            c = expression[multiindex]
-            return Indexed(*c.ufl_operands) if isinstance(c, Indexed) else c
+            return expression[multiindex]
         except ValueError:
-            return Operator.__new__(cls)
+            self = Operator.__new__(cls)
+            self._initialised = False
+            return self
 
     def __init__(self, expression, multiindex):
         """Initialise."""
+        if self._initialised:
+            return
         # Store operands
         Operator.__init__(self, (expression, multiindex))
 
@@ -81,7 +85,7 @@ def __init__(self, expression, multiindex):
         efi = expression.ufl_free_indices
         efid = expression.ufl_index_dimensions
         fi = list(zip(efi, efid))
-        for pos, ind in enumerate(multiindex._indices):
+        for pos, ind in enumerate(multiindex):
             if isinstance(ind, Index):
                 fi.append((ind.count(), shape[pos]))
         fi = unique_sorted_indices(sorted(fi))
@@ -93,6 +97,7 @@ def __init__(self, expression, multiindex):
         # Cache free index and dimensions
         self.ufl_free_indices = fi
         self.ufl_index_dimensions = fid
+        self._initialised = True
 
     ufl_shape = ()