Nonlinear expressions illegally modify their children

[odow]

Found by lanl-ansi/PowerModelsDistribution.jl#459 (comment)

I don't know how we have't come across this before:

julia> using JuMP

julia> model = Model();

julia> @variable(model, x[1:2]);

julia> y = [NonlinearExpr(:+, Any[xi]) for xi in x]
2-element Vector{NonlinearExpr}:
 +(x[1])
 +(x[2])

julia> a = @expression(model, sum(y[i] for i in 1:2))
x[1] + x[2]

julia> y
2-element Vector{NonlinearExpr}:
 x[1] + x[2]
 +(x[2])

Note that y[1] has been modified.

[odow]

This is the issue:

julia> using JuMP

julia> import JuMP._MA as MA

julia> model = Model();

julia> @variable(model, x);

julia> y = NonlinearExpr(:+, Any[x])
+(x)

julia> a = MA.operate!!(MA.add_mul, MA.Zero(), y)
+(x)

julia> a === y
true