Allow creation of polynomial ring over lazy power series ring

src/sage/rings/lazy_series.py

@@ -221,7 +221,7 @@
 #                  https://www.gnu.org/licenses/
 # ****************************************************************************
 
-from sage.structure.element import Element, parent
+from sage.structure.element import Element, ModuleElement, RingElement, parent
 from sage.structure.richcmp import op_EQ, op_NE
 from sage.misc.misc_c import prod
 from sage.arith.power import generic_power
@@ -262,7 +262,7 @@
 )
 
 
-class LazyModuleElement(Element):
+class LazyModuleElement(ModuleElement):
     r"""
     A lazy sequence with a module structure given by term-wise
     addition and scalar multiplication.
@@ -2928,7 +2928,7 @@ def sqrt(self):
         return self ** QQ((1, 2))  # == 1/2
 
 
-class LazyCauchyProductSeries(LazyModuleElement):
+class LazyCauchyProductSeries(LazyModuleElement, RingElement):
     r"""
     A class for series where multiplication is the Cauchy product.
 
@@ -2945,6 +2945,13 @@ class LazyCauchyProductSeries(LazyModuleElement):
         sage: f = 1 / (1 - z)
         sage: f
         1 + z + z^2 + O(z^3)
+
+    TESTS:
+
+    Check creation of polynomial ring over lazy power series ring::
+
+        sage: R.<x> = LazyPowerSeriesRing(QQ)
+        sage: T.<t> = R[]
     """
     def valuation(self):
         r"""

src/sage/symbolic/expression.pyx

@@ -13733,6 +13733,7 @@ cpdef new_Expression(parent, x):
 
     from sage.rings.infinity import (infinity, minus_infinity,
                                      unsigned_infinity)
+    from sage.rings.lazy_series import LazyCauchyProductSeries
     from sage.structure.factorization import Factorization
     from sage.categories.sets_cat import Sets
 
@@ -13757,7 +13758,8 @@ cpdef new_Expression(parent, x):
         return new_Expression_from_GEx(parent, g_mInfinity)
     elif x is unsigned_infinity:
         return new_Expression_from_GEx(parent, g_UnsignedInfinity)
-    elif isinstance(x, (RingElement, Matrix)):
+    elif isinstance(x, (RingElement, Matrix)) and not isinstance(x, LazyCauchyProductSeries):
+        # TODO allow conversion from LazyCauchyProductSeries to SR results in some mysterious bug
         if x.parent().characteristic():
             raise TypeError('positive characteristic not allowed in symbolic computations')
         exp = x