DNM: Fix and test support for Σ, ∑,and ∏

[odow]

Closes #3533

We've always supported Σ, ∑,and ∏, but these weren't tested, and the only worked for generators, not with a single itterable argument.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4a716c3) 98.25% compared to head (2830c07) 98.25%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3536   +/-   ##
=======================================
  Coverage   98.25%   98.25%           
=======================================
  Files          37       37           
  Lines        5563     5569    +6     
=======================================
+ Hits         5466     5472    +6     
  Misses         97       97           

Files	Coverage Δ
src/macros.jl	98.02% <100.00%> (+0.01%)	⬆️

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[blegat]

Is that an issues with MutableArithmetics, it seems that it is always using _is_sum

[odow]

The generator case is handled here:
https://github.com/jump-dev/MutableArithmetics.jl/blob/37bb9bfc792dd305ef971992c601ebea9f9812e3/src/rewrite_generic.jl#L76

But maybe we shouldn't support this. \sum and \Sigma aren't defined objects in Julia, so it will conflict if someone defines their own \sum function.

The problem is really that MutableArithmetics should not have tried to be clever with unicode

[odow]

Doc failure is #3538

[blegat]

I see, when it's going through this code then it works but when it's just gets executed as is then it fails.
I guess the issue with this PR is if the user does

julia> Σ = 1
1

and then uses that in its macro. Then if you replace it with sum, that's an issue because it's not 1 anymore.
Maybe MA shouldn't do what it does. I think that comes from a copy-paste of the old rewriting code from JuMP.

[odow]

Maybe MA shouldn't do what it does. I think that comes from a copy-paste of the old rewriting code from JuMP.

Yeah

[odow]

I think I'm going to close this as won't-fix. The \Sigma isn't documented, so it's mostly a legacy quirk. We should encourage people not to use it.