From 2b40d390c2d554e02192dd6fda48caa6264ba5d7 Mon Sep 17 00:00:00 2001
From: Travis Scrimshaw <tcscrims at gmail.com>
Date: Sun, 26 Sep 2021 15:18:58 +0900
Subject: [PATCH] Strengthening Laurent series conversion.

---
 src/sage/rings/lazy_series_ring.py | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/src/sage/rings/lazy_series_ring.py b/src/sage/rings/lazy_series_ring.py
index f68dbecac83..35e1c921dfc 100644
--- a/src/sage/rings/lazy_series_ring.py
+++ b/src/sage/rings/lazy_series_ring.py
@@ -34,6 +34,7 @@
 from sage.rings.polynomial.laurent_polynomial_ring import LaurentPolynomialRing
 from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing
 from sage.rings.lazy_series import (LazyModuleElement,
+                                    LazyCauchyProductSeries,
                                     LazyLaurentSeries,
                                     LazyDirichletSeries)
 from sage.structure.global_options import GlobalOptions
@@ -253,6 +254,13 @@ def _element_constructor_(self, x=None, valuation=None, degree=None, constant=No
             ...
             ValueError: constant may only be specified if the degree is specified
 
+            sage: D = LazyDirichletSeriesRing(ZZ, 't')
+            sage: m = D(moebius)
+            sage: L(m)
+            Traceback (most recent call last):
+            ...
+            ValueError: unable to convert ... into Lazy Laurent Series Ring ...
+
         We support the old input format for ``constant``::
 
             sage: f = L(lambda i: i, valuation=-3, constant=-1, degree=3)
@@ -263,11 +271,6 @@ def _element_constructor_(self, x=None, valuation=None, degree=None, constant=No
             sage: f == g
             True
 
-            sage: D = LazyDirichletSeriesRing(ZZ, 't')
-            sage: m = D(moebius)
-            sage: L(m)
-            z - z^2 - z^3 - z^5 + z^6 - z^7 + O(z^8)
-
         .. TODO::
 
             Add a method to change the sparse/dense implementation.
@@ -318,7 +321,7 @@ def _element_constructor_(self, x=None, valuation=None, degree=None, constant=No
                 return self.element_class(self, coeff_stream)
 
             # Handle when it is a lazy series
-            if isinstance(x, LazyModuleElement):
+            if isinstance(x, LazyCauchyProductSeries):
                 if x._coeff_stream._is_sparse is not self._sparse:
                     # TODO: Implement a way to make a self._sparse copy
                     raise NotImplementedError("cannot convert between sparse and dense")