Updates for MOI 0.9

[mlubin]

Do not merge until MOI 0.9 is tagged.

[ccoffrin]

Should this be "~0.9"?

[mlubin]

Yes

[blegat]

Both "0.9" and "~0.9" are equivalent, no ?: https://julialang.github.io/Pkg.jl/v1/compatibility/
For "0.9", it's ok if you don't change the left-most nonzero so the interval is [0.9, 0.10).
For "~0.9"~, it's ok if you only modify patch so the interval is [0.9, 0.10)` as well.

[mlubin]

Caret specifiers are the default and hence 1.2.3 == ^1.2.3
PkgB = "^1.2" # [1.2.0, 2.0.0)

If I understand correctly, "0.9" would allow anything up to 1.0.

[blegat]

A caret specifier allows upgrade that would be compatible according to semver. An updated dependency is considered compatible if the new version does not modify the left-most non zero digit in the version specifier.

For "1.2.3", the left-most is "1" so "1.3" does not modify the left-most nonzero.
For "0.9", the left-most is "9" so "0.10" does modify the left-most nonzero.

[Wikunia]

According to jump-dev/MathOptInterface.jl#736
DualObjectiveValue should be implemented which isn't the case as it's not supported.
Therefore the test config is changed. Is this the way it should be handled or should it return nothing or something like that? The requirements read like the function should exist to support MOI v0.9

[blegat]

This was discussed on Gitter on June 03.
The guideline if it's not implemented by a solver is to use MOIU.get_fallback. In this case, to quote @mlubin in Gitter (June 03 13h31):

The bigger issue is that dual objective is not a well-defined concept for NLP solvers in general, as far as I'm aware. We can compute it manually if all constraints are linear, but that's about it.

So we could implement it and call MOIU.get_fallback if the problem is linear the throw an error if it's not.

[mlubin]

I don't expect to have time to tidy this up and merge before the weekend, so it's up for grabs until then.

[blegat]

@mlubin Good to merge and release as Ipopt v0.6.1 ?

[mlubin]

This should be removed

[mlubin]

Yes

[ccoffrin]

Thanks Miles!!!