Disallow mixing of pos.char.ring elements and symbolic variables

[rwst]

This is to have a place for code examples and discussion of impact of the proposal in https://groups.google.com/forum/?hl=en#!topic/sage-devel/auLwQWtpnz0

The following produces a segmentation fault (Sage 7.4, Ubuntu 14.04 or OS X 10.10.5),
see Ask Sage.

sage: F = GF(3)
sage: num, den = F(2)*x + F(1), x
sage: num/den
 ... segmentation fault ...

With this branch this gives: TypeError: Multiplication of symbolic variable and an element of a ring with positive characteristic.

See also sage-support.

CC: @jdemeyer @videlec @slel

Component: symbolics

Keywords: segfault

Branch/Commit: u/rws/disallow_mixing_of_pos_char_ring_elements_and_symbolic_variables @ 8de8398

Reviewer: Ralf Stephan

Issue created by migration from https://trac.sagemath.org/ticket/21391

[rwst]

Branch: u/rws/disallow_mixing_of_pos_char_ring_elements_and_symbolic_variables

[rwst]

comment:2

In a first scan these doctests would have to be changed:

sage -t src/sage/symbolic/expression.pyx  # 11 doctests failed
sage -t src/sage/symbolic/pynac.pyx  # 2 doctests failed
sage -t src/sage/rings/polynomial/polynomial_element.pyx  # 2 doctests failed
sage -t src/sage/rings/polynomial/laurent_polynomial.pyx  # 2 doctests failed
sage -t src/sage/rings/polynomial/polynomial_zz_pex.pyx  # 1 doctest failed

---

New commits:

7147699

21391: Disallow mixing of pos.char.ring elements and symbolic variables

[rwst]

Commit: 7147699

[rwst]

Author: Ralf Stephan

[rwst]

comment:3

Alternatively this could be made a stopgap. Which is better?

[slel]

Changed keywords from none to segfault

[slel]

comment:4

Adding an example leading to a segmentation fault to the ticket description.

[slel]

Description changed:

--- 
+++ 
@@ -1,3 +1,12 @@
 This is to have a place for code examples and discussion of impact of the proposal in https://groups.google.com/forum/?hl=en#!topic/sage-devel/auLwQWtpnz0
 
+The following produces a segmentation fault (Sage 7.4, Ubuntu 14.04 or OS X 10.10.5),
+see [Ask Sage](https://ask.sagemath.org/question/35770/).
 
+```
+sage: F = GF(3)
+sage: num, den = F(2)*x + F(1), x
+sage: num/den
+ ... segmentation fault ...
+```
+

[slel]

Description changed:

--- 
+++ 
@@ -10,3 +10,4 @@
  ... segmentation fault ...
 ```
 
+See also [sage-support](https://groups.google.com/d/topic/sage-support/vWghGEC_ANg/discussion).

[rwst]

Description changed:

--- 
+++ 
@@ -10,4 +10,8 @@
  ... segmentation fault ...
 ```
 
+With this branch this gives: `TypeError: Multiplication of symbolic variable and an element of a ring with positive characteristic.`
+
 See also [sage-support](https://groups.google.com/d/topic/sage-support/vWghGEC_ANg/discussion).
+
+