Trac #25296: RingConverter should handle named variables by default

In the current version of Sage, when a !RingConverter is used to convert
from the symbolic ring to an algebraic ring, all converted variables
must be specified in the !RingConverter's dictionary.  For example,

{{{
sage: from sage.symbolic.expression_conversions import RingConverter
sage: R = QQ['x']
sage: RC = RingConverter(R, {x: R.0})
sage: RC(x)
x
}}}

Attempting to convert without a dictionary produces an error:

{{{
sage: RC = RingConverter(R)
sage: RC(x)
...
TypeError:
}}}

This patch changes this behavior so that if a variable is not found in
the !RingConverter's dictionary, an attempt is make to convert it to the
ring using the ring's element constructor.  This allows the last example
to work as expected; the symbolic variable "x" is converted to the ring
element of the same name.

{{{
sage: from sage.symbolic.expression_conversions import RingConverter
sage: R = QQ['x']
sage: RC = RingConverter(R)
sage: R(x)
x
sage: x.parent()
Symbolic Ring
sage: R(x).parent()
Univariate Polynomial Ring in x over Rational Field
}}}

URL: https://trac.sagemath.org/25296
Reported by: gh-BrentBaccala
Ticket author(s): Brent Baccala
Reviewer(s): Marc Mezzarobba

Author: Release Manager

src/sage/symbolic/expression_conversions.py

@@ -1801,8 +1801,9 @@ def __init__(self, R, subs_dict=None):
 
     def symbol(self, ex):
         """
-        All symbols appearing in the expression must appear in *subs_dict*
-        in order for the conversion to be successful.
+        All symbols appearing in the expression must either appear in *subs_dict*
+        or be convertable by the ring's element constructor in order for the
+        conversion to be successful.
 
         EXAMPLES::
 
@@ -1815,12 +1816,18 @@ def symbol(self, ex):
             sage: R(x+pi)
             Traceback (most recent call last):
             ...
-            TypeError
+            TypeError: unable to simplify to a real interval approximation
+
+            sage: R = RingConverter(QQ['x'])
+            sage: R(x^2+x)
+            x^2 + x
+            sage: R(x^2+x).parent()
+            Univariate Polynomial Ring in x over Rational Field
         """
         try:
             return self.ring(self.subs_dict[ex])
         except KeyError:
-            raise TypeError
+            return self.ring(ex)
 
     def pyobject(self, ex, obj):
         """