[Utilities] fix a Utilities.AbstractModel with no constraints (#2819)

Author: odow

src/Utilities/model.jl

@@ -369,10 +369,10 @@ end
 function MOI.get(
     model::AbstractModel{T},
     attr::MOI.ListOfConstraintTypesPresent,
-) where {T}
+)::Vector{Tuple{Type,Type}} where {T}
     return vcat(
-        MOI.get(model.constraints, attr)::Vector{Tuple{Type,Type}},
-        MOI.get(model.variables, attr)::Vector{Tuple{Type,Type}},
+        MOI.get(model.constraints, attr),
+        MOI.get(model.variables, attr),
     )
 end
 

src/Utilities/struct_of_constraints.jl

@@ -152,9 +152,10 @@ function MOI.get(
 end
 
 function MOI.is_empty(model::StructOfConstraints)
-    return mapreduce_constraints(&, model, true) do constrs
+    ret = mapreduce_constraints(&, model, true) do constrs
         return constrs === nothing || MOI.is_empty(constrs)
     end
+    return something(ret, true)
 end
 
 function MOI.empty!(model::StructOfConstraints)

test/Utilities/model.jl

@@ -617,6 +617,16 @@ function test_struct_of_constraints_by_set_types()
     return
 end
 
+MOI.Utilities.@model(EmptyModel, (), (), (), (), (), (), (), ())
+
+function test_empty_model()
+    model = EmptyModel{Float64}()
+    @test MOI.is_empty(model)
+    @test MOI.get(model, MOI.ListOfConstraintTypesPresent()) ==
+          Tuple{Type,Type}[]
+    return
+end
+
 end  # module
 
 TestModel.runtests()