gh-35045: Convert result of multivariate polynomial evaluation into correct parent
    
Importing trac/u/roed/incorrect_parent_when_evaluating_constant_multivar
iate_polynomial...

(meant to fix #33373)
    
URL: #35045
Reported by: Marc Mezzarobba
Reviewer(s): Marc Mezzarobba, Travis Scrimshaw

Author: Release Manager

src/sage/modular/modform_hecketriangle/graded_ring_element.py

@@ -1566,7 +1566,9 @@ def _q_expansion_cached(self, prec, fix_d, subs_d, d_num_prec, fix_prec = False)
         Y  = SC.f_i_ZZ().base_extend(formal_d.parent())
 
         if (self.parent().is_modular()):
-            qexp = self._rat.subs(x=X, y=Y, d=formal_d)
+            # z does not appear in self._rat but we need to specialize it for
+            # the evaluation to land in the correct parent
+            qexp = self._rat.subs(x=X, y=Y, z=0, d=formal_d)
         else:
             Z = SC.E2_ZZ().base_extend(formal_d.parent())
             qexp = self._rat.subs(x=X, y=Y, z=Z, d=formal_d)

src/sage/rings/polynomial/multi_polynomial_libsingular.pyx

@@ -2057,7 +2057,18 @@ cdef class MPolynomial_libsingular(MPolynomial):
             9
             sage: a.parent() is QQ
             True
+
+        See :trac:`33373`::
+
+            sage: k.<a> = GF(2^4)
+            sage: R.<x> = PolynomialRing(k, 1)
+            sage: f = R(1)
+            sage: S.<y> = PolynomialRing(k, 1)
+            sage: f(y).parent()
+            Multivariate Polynomial Ring in y over Finite Field in a of size 2^4
         """
+        cdef Element sage_res
+
         if len(kwds) > 0:
             f = self.subs(**kwds)
             if len(x) > 0:
@@ -2076,29 +2087,28 @@ cdef class MPolynomial_libsingular(MPolynomial):
         if l != parent._ring.N:
             raise TypeError("number of arguments does not match number of variables in parent")
 
+        res_parent = coercion_model.common_parent(parent._base, *x)
+        cdef poly *res    # ownership will be transferred to us in the else block
         try:
             # Attempt evaluation via singular.
             coerced_x = [parent.coerce(e) for e in x]
         except TypeError:
             # give up, evaluate functional
-            y = parent.base_ring().zero()
+            sage_res = parent.base_ring().zero()
             for (m,c) in self.dict().iteritems():
-                y += c*mul([ x[i]**m[i] for i in m.nonzero_positions()])
-            return y
-
-        cdef poly *res    # ownership will be transferred to us in the next line
-        singular_polynomial_call(&res, self._poly, _ring, coerced_x, MPolynomial_libsingular_get_element)
-        res_parent = coercion_model.common_parent(parent._base, *x)
-
-        if res == NULL:
-            return res_parent(0)
-        if p_LmIsConstant(res, _ring):
-            sage_res = si2sa( p_GetCoeff(res, _ring), _ring, parent._base )
-            p_Delete(&res, _ring)            # sage_res contains copy
+                sage_res += c * mul([x[i] ** m[i] for i in m.nonzero_positions()])
         else:
-            sage_res = new_MP(parent, res)   # pass on ownership of res to sage_res
+            singular_polynomial_call(&res, self._poly, _ring, coerced_x, MPolynomial_libsingular_get_element)
+
+            if res == NULL:
+                return res_parent(0)
+            if p_LmIsConstant(res, _ring):
+                sage_res = si2sa( p_GetCoeff(res, _ring), _ring, parent._base )
+                p_Delete(&res, _ring)            # sage_res contains copy
+            else:
+                sage_res = new_MP(parent, res)   # pass on ownership of res to sage_res
 
-        if parent(sage_res) is not res_parent:
+        if sage_res._parent is not res_parent:
             sage_res = res_parent(sage_res)
         return sage_res
 

src/sage/schemes/riemann_surfaces/riemann_surface.py

@@ -2128,6 +2128,7 @@ def rigorous_line_integral(self, upstairs_edge, differentials, bounding_data):
         # CCzg is required to be known as we need to know the ring which the minpolys
         # lie in.
         CCzg, bounding_data_list = bounding_data
+        CCz = CCzg.univariate_ring(CCzg.gen(1)).base_ring()
 
         d_edge = tuple(u[0] for u in upstairs_edge)
         # Using a try-catch here allows us to retain a certain amount of back
@@ -2193,7 +2194,7 @@ def local_N(ct, rt):
                 z_1 = a0lc.abs() * prod((cz - r).abs() - rho_z for r in a0roots)
                 n = minpoly.degree(CCzg.gen(1))
                 ai_new = [
-                    (minpoly.coefficient({CCzg.gen(1): i}))(z=cz + self._CCz.gen(0))
+                    CCz(minpoly.coefficient({CCzg.gen(1): i}))(z=cz + self._CCz.gen(0))
                     for i in range(n)
                 ]
                 ai_pos = [self._RRz([c.abs() for c in h.list()]) for h in ai_new]

src/sage/schemes/toric/points.py

@@ -849,7 +849,7 @@ def inhomogeneous_equations(self, ring, nonzero_coordinates, cokernel):
         z = [ring.zero()] * nrays
         for i, value in zip(nonzero_coordinates, z_nonzero):
             z[i] = value
-        return [poly(z) for poly in self.polynomials]
+        return [poly.change_ring(ring)(z) for poly in self.polynomials]
 
     def solutions_serial(self, inhomogeneous_equations, log_range):
         """