Trac #31530: New subs bug caused by #30378

As reported in https://trac.sagemath.org/ticket/30378#comment:49

Observed:
{{{
sage: a, b, epsilon = SR.var('a, b, epsilon')
sage: (a + b*epsilon).series(epsilon, 2).subs(a=a, b=b)
b*epsilon
}}}

Expected:
{{{
sage: a, b, epsilon = SR.var('a, b, epsilon')
sage: (a + b*epsilon).series(epsilon, 2).subs(a=a, b=b)
(a) + (b)*x + Order(x^2)
}}}

URL: https://trac.sagemath.org/31530
Reported by: mkoeppe
Ticket author(s): Dave Morris
Reviewer(s): Matthias Koeppe

Author: Release Manager

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=371681ab50f9b1156175f71d26999923b68bd410
-md5=961d28fdae428e2b911db45f169e7296
-cksum=4212743206
+sha1=2916d50d525d1b099bb5c54c2dbc60156bd79ef3
+md5=f121b77604fd797ddda9c8098adabd63
+cksum=3390986155

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-9322acfe193c57e20d4d655e42e5ee39bc3f9666
+761840fd551cb290092340bc73917336f22adfec

build/pkgs/pynac/package-version.txt

@@ -1 +1 @@
-0.7.27.p1
+0.7.27.p3

build/pkgs/pynac/patches/too_much_sub.patch

@@ -0,0 +1,15 @@
+diff --git a/ginac/power.cpp b/ginac/power.cpp
+index 5c07324..837fce3 100644
+--- a/ginac/power.cpp
++++ b/ginac/power.cpp
+@@ -745,6 +745,10 @@ ex power::subs(const exmap & m, unsigned options) const
+         if (!are_ex_trivially_equal(basis, subsed_basis)
+ 	 || !are_ex_trivially_equal(exponent, subsed_exponent)) {
+ 		ex p = power(subsed_basis, subsed_exponent);
++		if (!is_exactly_a<power>(p)) {
++		    // trac 30378 and 31530: do not over-substitute
++	        return p;
++	    }
+                 ex t = ex_to<power>(p).subs_one_level(m, options);
+                 if ((t-*this).is_zero())
+                         return p;

src/sage/symbolic/expression.pyx

@@ -5489,6 +5489,12 @@ cdef class Expression(CommutativeRingElement):
             sage: f = function("f")
             sage: integrate(f(x), x, 0, a).subs(a=cos(a))
             integrate(f(x), x, 0, cos(a))
+
+        Check that :trac:`31530` is fixed::
+
+            sage: a, b = var("a b")
+            sage: (a + b*x).series(x, 2).subs(a=a, b=b)
+            (a) + (b)*x + Order(x^2)
         """
         cdef dict sdict = {}
         cdef GEx res
@@ -5509,28 +5515,6 @@ cdef class Expression(CommutativeRingElement):
             # Check for duplicate
             _dict_update_check_duplicate(sdict, varkwds)
 
-        # To work around the pynac bug in :trac:`30378`, we use two steps to do a
-        # substitution that only involves plugging expressions into variables, but
-        # where some of the expressions include variables that are in self.
-        if all(self.parent(k).is_symbol() for k in sdict.keys()):
-            dict_vars = tuple(v for k in sdict.keys()
-                for v in self.parent(sdict[k]).variables())
-            if not set(self.variables()).isdisjoint(dict_vars):
-                # Step 1: replace each variable with a new temporary variable
-                temp_vars = {v : self.parent().symbol() for v in self.variables()}
-                with hold:
-                    first_step = self.substitute(temp_vars)
-                # Step 2: make the original substitutions into the new variables
-                result = first_step.substitute({
-                    temp_vars[v] :
-                    sdict[v] if v in sdict.keys() else v
-                    for v in self.variables()})
-                if not set(result.variables()).issubset(self.variables() + dict_vars):
-                    raise RuntimeError("substitution failed")
-                return result
-
-        # We are not in the basic case of only substituting expressions into
-        # variables, so we ask Ginac to do the work.
         cdef GExMap smap
         for k, v in sdict.iteritems():
             smap.insert(make_pair((<Expression>self.coerce_in(k))._gobj,

src/sage/tests/books/computational-mathematics-with-sagemath/integration_doctest.py

@@ -149,7 +149,7 @@
   sage: mpmath.quad(sin(sin(x)), [0, 1])
   Traceback (most recent call last):
   ...
-  TypeError: unable to convert mpf('0.5') to a symbolic expression
+  TypeError: no canonical coercion from <type 'sage.libs.mpmath.ext_main.mpf'> to Symbolic Ring
 
 Sage example in ./integration.tex, line 866::