Support MA.Zero in objective and constraints (jump-dev#2219)

* Support MA.Zero in objective and cosntraints

* Update macros.jl

src/macros.jl

@@ -245,6 +245,11 @@ function build_constraint(_error::Function, α::Number,
                           set::MOI.AbstractScalarSet)
     return build_constraint(_error, convert(AffExpr, α), set)
 end
+function build_constraint(
+    _error::Function, ::MutableArithmetics.Zero, set::MOI.AbstractScalarSet
+)
+    return build_constraint(_error, zero(AffExpr), set)
+end
 
 function build_constraint(_error::Function, x::Vector{<:AbstractJuMPScalar},
                           set::MOI.AbstractVectorSet)
@@ -662,7 +667,7 @@ Adds multiple variables to model at once, in the same fashion as `@variable` mac
     end)
 
 !!! note
-    Keyword arguments must be contained within parentheses (refer to the example above). 
+    Keyword arguments must be contained within parentheses (refer to the example above).
 """ :(@variables)
 
 """

src/objective.jl

@@ -13,8 +13,8 @@
     relative_gap(model::Model)
 
 Return the final relative optimality gap after a call to `optimize!(model)`.
-Exact value depends upon implementation of MathOptInterface.RelativeGap() 
-by the particular solver used for optimization. 
+Exact value depends upon implementation of MathOptInterface.RelativeGap()
+by the particular solver used for optimization.
 """
 function relative_gap(model::Model)::Float64
     return MOI.get(model, MOI.RelativeGap())
@@ -124,6 +124,12 @@ function set_objective(
     set_objective_function(model, func)
 end
 
+function set_objective(
+    model::Model, sense::MOI.OptimizationSense, ::MutableArithmetics.Zero
+)
+    set_objective(model, sense, 0.0)
+end
+
 function set_objective(model::Model, sense::MOI.OptimizationSense, func)
     error("The objective function `$(func)` is not supported by JuMP.")
 end

test/macros.jl

@@ -54,6 +54,15 @@ end
     @test constraint_object(c).set == MOI.EqualTo(0.0)
 end
 
+@testset "MutableArithmetics.Zero (Issue #2087)" begin
+    model = Model()
+    @objective(model, Min, sum(1 for _ in 1:0))
+    @test objective_function(model) == AffExpr(0.0)
+    c = @constraint(model, sum(1 for _ in 1:0) in MOI.EqualTo(0.0))
+    @test constraint_object(c).func == AffExpr(0.0)
+    @test constraint_object(c).set == MOI.EqualTo(0.0)
+end
+
 mutable struct MyVariable
     test_kw::Int
     info::JuMP.VariableInfo