From d628f1459d21ecd34c14366336a7b56fc677b78c Mon Sep 17 00:00:00 2001
From: Salvatore Stella <sstella@ncsu.edu>
Date: Tue, 25 Dec 2018 20:09:34 +0100
Subject: [PATCH] Fix 26958

---
 .../rings/polynomial/multi_polynomial_ring_base.pxd   |  2 ++
 .../rings/polynomial/multi_polynomial_ring_base.pyx   | 11 +++++++++++
 src/sage/rings/polynomial/polynomial_element.pyx      |  2 +-
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/sage/rings/polynomial/multi_polynomial_ring_base.pxd b/src/sage/rings/polynomial/multi_polynomial_ring_base.pxd
index 7e713c01dac..87738032ad6 100644
--- a/src/sage/rings/polynomial/multi_polynomial_ring_base.pxd
+++ b/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
diff --git a/src/sage/rings/polynomial/multi_polynomial_ring_base.pyx b/src/sage/rings/polynomial/multi_polynomial_ring_base.pyx
index ee76f34c922..fdd4ce930ea 100644
--- a/src/sage/rings/polynomial/multi_polynomial_ring_base.pyx
+++ b/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):
diff --git a/src/sage/rings/polynomial/polynomial_element.pyx b/src/sage/rings/polynomial/polynomial_element.pyx
index 840810f42e5..aeb57b27fb5 100644
--- a/src/sage/rings/polynomial/polynomial_element.pyx
+++ b/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
     """