From d33ee66d85d31c78844f5cbf23d6541e91f5ffb6 Mon Sep 17 00:00:00 2001
From: Fabio Luporini <fabio@devitocodes.com>
Date: Thu, 4 Jun 2026 13:44:34 +0100
Subject: [PATCH] compiler: Fix DefFunction (re)construction

---
 devito/symbolics/extended_sympy.py |  9 ++++++++-
 tests/test_symbolics.py            | 30 ++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/devito/symbolics/extended_sympy.py b/devito/symbolics/extended_sympy.py
index 4523fbd2f4..41e6ed7547 100644
--- a/devito/symbolics/extended_sympy.py
+++ b/devito/symbolics/extended_sympy.py
@@ -9,6 +9,7 @@
 import sympy
 from sympy import Expr, Function, Number, Tuple, cacheit, sympify
 from sympy.core.decorators import call_highest_priority
+from sympy.core.function import Application
 from sympy.logic.boolalg import BooleanFunction
 
 from devito.finite_differences.elementary import Max, Min
@@ -718,7 +719,13 @@ def __new__(cls, name, arguments=None, template=None, **kwargs):
         if _template:
             args.append(Tuple(*_template))
 
-        obj = Function.__new__(cls, *args)
+        # `Function.__new__` and `Application.__new__` are both cached by
+        # SymPy. DefFunction subclasses may attach reconstruction kwargs as
+        # side attributes after this base constructor returns; going through
+        # the cached route could then alias a previous object and mutate it
+        # during reconstruction. Call Application's uncached constructor
+        # explicitly instead of using super()/Function.__new__.
+        obj = Application.__new__.__wrapped__(cls, *args)
         obj._name = name
         obj._arguments = tuple(_arguments)
         obj._template = tuple(_template)
diff --git a/tests/test_symbolics.py b/tests/test_symbolics.py
index 48709d7cd1..5af78117a2 100644
--- a/tests/test_symbolics.py
+++ b/tests/test_symbolics.py
@@ -934,6 +934,36 @@ def __new__(cls, name=None, arguments=None, p0=None, p1=None, p2=None):
         assert func1.p1 == (g,)
         assert func1.p2 == 'bar'
 
+    def test_custom_def_function_reconstruction_no_aliasing(self):
+
+        class MyDefFunction(DefFunction):
+            __rargs__ = ('name', 'arguments')
+            __rkwargs__ = ('p0',)
+
+            def __new__(cls, name=None, arguments=None, p0=None):
+                obj = super().__new__(cls, name=name, arguments=arguments)
+                obj.p0 = p0
+                return obj
+
+            def _hashable_content(self):
+                return super()._hashable_content() + (self.p0,)
+
+        grid = Grid(shape=(4, 4))
+
+        f = Function(name='f', grid=grid)
+        g = Function(name='g', grid=grid)
+
+        func0 = MyDefFunction(name='foo', arguments=f.indexify(), p0=f)
+        h0 = hash(func0)
+
+        func1 = func0.func(p0=g)
+
+        assert func1 is not func0
+        assert func1 != func0
+        assert hash(func0) == h0
+        assert func0.p0 is f
+        assert func1.p0 is g
+
     def test_reduce_to_number(self):
         grid = Grid(shape=(4, 4))
         x, _ = grid.dimensions