Fix sympify

Author: isuruf

symengine/lib/symengine_wrapper.pyx

@@ -4069,7 +4069,7 @@ def diff(expr, *args):
         # Don't sympify matrices so that mutable matrices
         # return mutable matrices
         return _diff(expr, *args)
-    return _diff(sympify(expr, *args))
+    return _diff(sympify(expr), *args)
 
 
 def _diff(expr, *args):

symengine/tests/test_matrices.py

@@ -658,7 +658,9 @@ def test_cross():
 def test_diff():
     x = symbols("x")
     M = DenseMatrix(1, 2, [x**2, x])
-    assert M.diff(x) == DenseMatrix(1, 2, [2*x, 1])
+    result = M.diff(x)
+    assert isinstance(result, DenseMatrix)
+    assert result == DenseMatrix(1, 2, [2*x, 1])
 
 
 def test_immutablematrix():