Assignment allowed in macros where it shouldn't be

[mlubin]

Simplified from https://discourse.julialang.org/t/how-to-print-the-values-of-constraints/40040:

julia> m = Model();

julia> @variable(m, x);

julia> @constraint(m, (a = x) <= 1)
x ≤ 1.0

I'm not sure how exactly, but this should produce an error.

[blegat]

Why shouldn't it be allowed, this just stes the value of a as expected. The macro rewrites the expression to exploit mutability, nothing more.

[odow]

Agreed with @blegat. The macro effectively expands to

ss0 = JuMP.MutableArithmetics.Zero()
ss1 = JuMP.MutableArithmetics.operate!(
    JuMP.MutableArithmetics.add_mul, 
    ss0, 
    $(Expr(:(=), :a, :x))
)
ss2 = JuMP.MutableArithmetics.operate!(
    JuMP.MutableArithmetics.sub_mul, 
    ss1, 
    1,
)

The interpolation evaluates a = x, returns the RHS x, substitutes it in, and away we go. So we can't intercept it at a functional level.

I guess you could walk the expression of the constraint and see if there are any :(=), then throw an error, but it would get tricky to work around generators etc.

[odow]

I bet there is also at least one person relying on this with some weird expression, so erroring on this would break code.

julia> model = Model();

julia> @variable(model, x);

julia> @constraint(model, (a = 2; a^2) * x <= 1)
4 x ≤ 1.0

I vote we close this, because there's nothing technically wrong (and people may be relying on it), and it has only come up once on Discourse. We can revisit if there are more reports.

[odow]

Closing this as won't-fix for the reasons above. We can re-open if it comes up again.