TensorField.__call__ alters zero

[mjungmath]

sage: M = Manifold(3, 'M', start_index=1)
sage: X.<x,y,z> = M.chart()
sage: omega = M.diff_form(1, 'omega')
sage: omega[:] = [0, 0, 0]
sage: omega(X.frame()[1])
Scalar field omega(d/dx) on the 3-dimensional differentiable manifold M
sage: M.zero_scalar_field()
Scalar field omega(d/dx) on the 3-dimensional differentiable manifold M

Depends on #30266
Depends on #30291

CC: @egourgoulhon @tscrim @mkoeppe

Component: manifolds

Author: Michael Jung

Branch/Commit: u/gh-mjungmath/tensorfield___call___alters_zero @ 9f63351

Reviewer: Eric Gourgoulhon

Issue created by migration from https://trac.sagemath.org/ticket/30239

[mjungmath]

Branch: u/gh-mjungmath/tensorfield___call___alters_zero

[mjungmath]

Commit: 51ba4e3

[mjungmath]

New commits:

51ba4e3

Trac #30239: comp zero check for __call__

[mkoeppe]

Author: Michael Jung

[egourgoulhon]

comment:4

Good catch!

However the proposed fix

            if omega == 0 or vv == 0:
                return self.base_ring().zero()

is not acceptable, because the comparison with 0 can be very slow for complicated expressions.
Also, this does not fix the root of the bug, which results from the += operator acting on the zero element. It should be fixed per se, because it can lead to other errors.
FWIW, this bug was introduced in Sage 9.0 (everything is OK in earlier versions of Sage).

[egourgoulhon]

Changed author from Michael Jung to none

[egourgoulhon]

comment:5

Sorry the author name was deleted by mistake.

[egourgoulhon]

Author: Michael Jung