Fix 26958

src/sage/rings/polynomial/multi_polynomial_ring_base.pxd

@@ -1,7 +1,9 @@
 cimport sage.rings.ring
 from sage.structure.parent cimport Parent
+from cpython cimport bool
 
 cdef class MPolynomialRing_base(sage.rings.ring.CommutativeRing):
+    cdef public bool _no_generic_basering_coercion
     cdef object __ngens
     cdef object __term_order
     cdef public object _has_singular

src/sage/rings/polynomial/multi_polynomial_ring_base.pyx

@@ -63,6 +63,14 @@ cdef class MPolynomialRing_base(sage.rings.ring.CommutativeRing):
             True
             sage: A1(a) in A2
             True
+
+        Check that :trac:`26958` is fixed::
+
+            sage: from sage.rings.polynomial.multi_polynomial_libsingular import MPolynomialRing_libsingular
+            sage: class Foo(MPolynomialRing_libsingular):
+            ....:     pass
+            sage: Foo(QQ, 2, ['x','y'], 'degrevlex')
+            Multivariate Polynomial Ring in x, y over Rational Field
         """
         if base_ring not in _CommutativeRings:
             raise TypeError("The base ring %s is not a commutative ring" % base_ring)
@@ -85,6 +93,9 @@ cdef class MPolynomialRing_base(sage.rings.ring.CommutativeRing):
         else:
             category = polynomial_default_category(base_ring.category(), n)
 
+        # Avoid calling __init_extra__ of Algebras(...).parent_class
+        self._no_generic_basering_coercion = True
+
         sage.rings.ring.Ring.__init__(self, base_ring, names, category=category)
 
     def is_integral_domain(self, proof = True):

src/sage/rings/polynomial/polynomial_element.pyx

@@ -11496,7 +11496,7 @@ cdef class PolynomialBaseringInjection(Morphism):
     By :trac:`9944`, there are now only very few exceptions::
 
         sage: PolynomialRing(QQ,names=[]).coerce_map_from(QQ)
-        Generic morphism:
+        Call morphism:
           From: Rational Field
           To:   Multivariate Polynomial Ring in no variables over Rational Field
     """