[Utilities] fix operate(vcat for VectorNonlinearFunction (#2682)

Author: odow

src/Utilities/operate.jl

@@ -991,7 +991,15 @@ function operate(
             append!(out, scalarize(a))
         end
     end
-    return MOI.VectorNonlinearFunction(out)
+    _to_new_snf(f::MOI.ScalarNonlinearFunction) = copy(f)
+    _to_new_snf(f) = convert(MOI.ScalarNonlinearFunction, f)
+    # We need to `copy` the ScalarNonlinearFunction rows here, everything else
+    # will be `convert(ScalarNonlinearFunction, row)` into a new object, but the
+    # ScalarNonlinearFunction rows won't be, so mutating the return value of
+    # this `operate` method will mutate the inputs. This _is_ what Base.vcat
+    # does, but it doesn't fit with the general assumption that
+    # `Utilities.operate(` returns a new object.
+    return MOI.VectorNonlinearFunction(_to_new_snf.(out))
 end
 
 ### 6a: operate(::typeof(imag), ::Type{T}, ::F)

test/Utilities/test_operate!.jl

@@ -399,6 +399,24 @@ function test_operate_5a()
     return
 end
 
+function test_operate_5a_VectorNonlinearFunction()
+    x = MOI.ScalarNonlinearFunction(:+, Any[])
+    f = MOI.VectorNonlinearFunction([x])
+    g = MOI.Utilities.operate(vcat, Float64, f, f, x)
+    rows = [MOI.ScalarNonlinearFunction(:+, Any[]) for _ in 1:3]
+    h = MOI.VectorNonlinearFunction(rows)
+    @test g ≈ h
+    push!(x.args, 0.0)
+    @test g ≈ h
+    @test g.rows[1] !== g.rows[2]
+    @test g.rows[1] !== g.rows[3]
+    @test g.rows[2] !== g.rows[3]
+    rows = [MOI.ScalarNonlinearFunction(:+, Any[0.0])]
+    f_new = MOI.VectorNonlinearFunction(rows)
+    @test f ≈ f_new
+    return
+end
+
 function test_operate_6a()
     T = Float64
     @test MOI.Utilities.operate(imag, T, _test_function((0.0, 0.0, 0.0))) ≈