core: avoid deep expanding in Mul._eval_nseries()

This finally fix diofant#1139
Closes sympy/sympy#21176

diofant/core/mul.py

@@ -1382,7 +1382,7 @@ def ndiv(a, b):
     def _eval_nseries(self, x, n, logx):
         from ..simplify import powsimp
         terms = [t.nseries(x, n=n, logx=logx) for t in self.args]
-        return powsimp(self.func(*terms).expand(), combine='exp', deep=True)
+        return powsimp(self.func(*terms).expand(deep=False), combine='exp', deep=True)
 
     def _eval_as_leading_term(self, x):
         return self.func(*[t.as_leading_term(x) for t in self.args])

diofant/tests/series/test_residues.py

@@ -1,7 +1,7 @@
 import pytest
 
-from diofant import (Function, I, Rational, Symbol, cot, exp, factorial, log,
-                     pi, residue, root, sin, sqrt, tanh)
+from diofant import (Function, I, Mul, Rational, Symbol, cot, exp, factorial,
+                     log, pi, residue, root, sin, sqrt, tan, tanh)
 from diofant.abc import a, s, x, z
 
 
@@ -85,3 +85,11 @@ def test_sympyissue_21177():
 
     assert residue(e1, x, pt) == ans
     assert residue(e2, x, pt) == ans
+
+
+def test_sympyissue_21176():
+    e = x**2*cot(pi*x)/(x**4 + 1)
+    pt = -sqrt(2)/2 - sqrt(2)*I/2
+    assert residue(e, x, pt) == Mul(Rational(1, 4), sqrt(2), 1/(1 + I),
+                                    1/tan(sqrt(2)*pi/2 + sqrt(2)*I*pi/2),
+                                    evaluate=False)

docs/release/notes-0.13.rst

@@ -95,3 +95,4 @@ These Sympy issues also were addressed:
 * :sympyissue:`21245`: laurent series Fibonacci generating fuction
 * :sympyissue:`11833`: error in limit involving exp, sinh and an assumption (maybe related to caching)
 * :sympyissue:`9127`: ntheory.AskEvenHandler.Mul is order-dependent
+* :sympyissue:`21176`: Incorrect residue of x**2*cot(pi*x)/(x**4 + 1)