Fix helpful error

[blegat]

Closes #1365

[odow]

Should we throw a proper exception type to distinguish the helpful error message from the un-helpful "S not defined"? Because this wasn't caught in tests as it still throws an error.

[mlubin]

Can't we just make this work?

if var isa Number && !(value isa Number)
     parse_one_operator_variable(_error, infoexpr, Val(reverse_sense(sense)), esc_nonconstant(var))
    return value
end

[blegat]

I don't understand why it was passing the tests since we check that its an ErrorException and S not defined should be an UndefVarError.

[odow]

The question is whether it is more or less confusing for users. Then you have to say why

@variable(m, 0 <= x)

works but

a = 0
@variable(m, a <= x)

I can see it leading to more confusion than "it never works"

[blegat]

Can't we just make this work?

We should at least throw a warning because it's going to fail badly when it's not constant.
Or we add in the FAQ the meaning of the weird error that is got when it's not constant. I am fine with both.

[mlubin]

The confusion always seems to be with

@variable(m, 0 <= x)

I've yet to see a complaint about

a = 0
@variable(m, a <= x)

[odow]

Okay lets change it.

[codecov]

Codecov Report

Merging #1366 into master will increase coverage by 0.06%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #1366      +/-   ##
==========================================
+ Coverage   89.44%   89.51%   +0.06%     
==========================================
  Files          24       24              
  Lines        3412     3413       +1     
==========================================
+ Hits         3052     3055       +3     
+ Misses        360      358       -2

Impacted Files	Coverage Δ
src/macros.jl	89.8% <57.14%> (+0.4%)	⬆️

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[blegat]

I have changed it. It now works instead of erroring :)

[mlubin]

This comment is out of date, so are the argument names.

[blegat]

I explains where the function arguments come from. We should probably rename to lhs rhs though

[blegat]

I have refactored it in two methods

[mlubin]

Make a separate test for this.