gh-37646: Fix compilation with -std=c++17

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Fixes #36840.

0c96d61: I don't think there should be
any issues with this, as it should be exactly what `cpdef` does under
the hood. That being said, I'd like someone knowledgable with Cython to
sanity check this.

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### ⌛ Dependencies

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N/A

URL: #37646
Reported by: Shorden
Reviewer(s): Matthias Köppe, Shorden

Author: Release Manager

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=25a360e4892317526249e50eaa51297891511d61
-md5=0fd34b91fc06199e6c3672ad969472ee
-cksum=1226224895
+sha1=461fc89608a23f5e06c75bdd0a7480527fc3fa66
+md5=94145a9c6e55496266631d06c3d7fdeb
+cksum=613494009

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-d13f6164a314f330071fe2a99112e78de03347f0
+e9456b2da71c424d3ff188d5bb75e3c60f42be16

src/sage/modular/arithgroup/farey.cpp

@@ -24,6 +24,7 @@
 #include <sstream>
 #include <algorithm>
 #include <cassert>
+#include <functional>
 
 #include <gmpxx.h>
 #include <Python.h>
@@ -737,7 +738,7 @@ size_t FareySymbol::nu3() const {
 size_t FareySymbol::rank_pi() const {
   if( index() == 2 ) return 1;
   return count_if(pairing.begin(), pairing.end(),
-                  bind2nd(greater<int>(), 0))/2;
+                  bind(greater<int>(), placeholders::_1, 0))/2;
 }
 
 size_t FareySymbol::number_of_cusps() const {

src/sage/modular/arithgroup/farey_symbol.pyx

@@ -1,4 +1,6 @@
 # distutils: sources = sage/modular/arithgroup/sl2z.cpp sage/modular/arithgroup/farey.cpp
+# distutils: language = c++
+# distutils: extra_compile_args = -std=c++11
 # sage.doctest: needs sage.libs.pari
 r"""
 Farey symbol for arithmetic subgroups of `\PSL_2(\ZZ)`

src/sage/rings/polynomial/hilbert.pyx

@@ -106,7 +106,7 @@ cdef inline list interred(list L) noexcept:
     # that appears later in L.
     if not L:
         return []
-    L.sort(key=ETuple.unweighted_degree)
+    L.sort(key=ETuple._unweighted_degree)
     cdef size_t i
     cdef ETuple m
     cdef list result = [<ETuple> PyList_GET_ITEM(L, 0)]

src/sage/rings/polynomial/polydict.pxd

@@ -14,6 +14,8 @@ cdef class ETuple:
     cdef ETuple _new(self) noexcept
     cdef int get_exp(self, size_t i) noexcept
 
+    # need a cdef version for function pointers
+    cdef int _unweighted_degree(self) except *
     cpdef int unweighted_degree(self) except *
     cpdef int weighted_degree(self, tuple w) except *
     cpdef int unweighted_quotient_degree(self, ETuple other) except *

src/sage/rings/polynomial/polydict.pyx

@@ -1845,7 +1845,7 @@ cdef class ETuple:
 
     # additional methods
 
-    cpdef int unweighted_degree(self) except *:
+    cdef int _unweighted_degree(self) except *:
         r"""
         Return the sum of entries.
 
@@ -1863,6 +1863,20 @@ cdef class ETuple:
             degree += self._data[2 * i + 1]
         return degree
 
+    cpdef int unweighted_degree(self) except *:
+        r"""
+        Return the sum of entries.
+
+        EXAMPLES::
+
+             sage: from sage.rings.polynomial.polydict import ETuple
+             sage: ETuple([1, 1, 0, 2, 0]).unweighted_degree()
+             4
+             sage: ETuple([-1, 1]).unweighted_degree()
+             0
+        """
+        return self._unweighted_degree()
+
     @cython.boundscheck(False)
     @cython.wraparound(False)
     cpdef int weighted_degree(self, tuple w) except *: