[FileFormats.NL] fix writing free nonlinear constraint (#2437)

Author: odow

src/FileFormats/NL/NL.jl

@@ -690,9 +690,8 @@ function Base.write(io::IO, model::Model)
                 println(io, " ", _str(g.upper))
             elseif g.opcode == 2
                 println(io, " ", _str(g.lower))
-                # Free constraints aren't supported for nonlinear.
-                # elseif g.opcode == 3
-                #     println(io)
+            elseif g.opcode == 3
+                println(io)
             else
                 @assert g.opcode == 4
                 println(io, " ", _str(g.lower))

test/FileFormats/NL/NL.jl

@@ -1090,6 +1090,17 @@ function test_empty_constraint()
     @test haskey(mapping, ci)
 end
 
+function test_writing_free_constraint()
+    model = MOI.Utilities.UniversalFallback(MOI.Utilities.Model{Float64}())
+    x = MOI.add_variable(model)
+    f = MOI.ScalarNonlinearFunction(:log, Any[x])
+    MOI.add_constraint(model, f, MOI.Interval(-Inf, Inf))
+    n = NL.Model()
+    _ = MOI.copy_to(n, model)
+    @test occursin("r\n3\nb", sprint(write, n))
+    return
+end
+
 function test_malformed_constraint_error()
     model = MOI.Utilities.UniversalFallback(MOI.Utilities.Model{Float64}())
     x = MOI.add_variable(model)