Don't falsely claim that Integer variables are supported

They really are not supported, and the claim is only there
to provide a nice diagnostic.

But that prevents MOI `IntegerToZeroOneBridge` from triggering,
and providing Alpine with transparent support for them.

Fixes #244

examples/MINLPs/milp.jl

@@ -1,7 +1,9 @@
 function milp(; solver = nothing)
     m = JuMP.Model(solver)
 
-    @variable(m, 0 <= objvar <= 20, Int)
+    @variable(m, objvar, Int)
+    #@variable(m, 0 <= objvar <= 20, Int)
+    @constraint(m, objvar in MOI.Interval(0.0, 20.0))
     @objective(m, Min, objvar)
 
     return m

src/MOI_wrapper/MOI_wrapper.jl

@@ -322,18 +322,6 @@ function MOI.add_constraint(model::Optimizer, vi::MOI.VariableIndex, set::SCALAR
     return MOI.ConstraintIndex{typeof(vi),typeof(set)}(vi.value)
 end
 
-function MOI.supports_constraint(
-    ::Optimizer,
-    ::Type{MOI.VariableIndex},
-    ::Type{MOI.Integer},
-)
-    return true
-end
-
-function MOI.add_constraint(model::Optimizer, f::MOI.VariableIndex, set::MOI.Integer)
-    model.var_type_orig[f.value] = :Int
-    return MOI.ConstraintIndex{typeof(f),typeof(set)}(f.value)
-end
 function MOI.supports_constraint(
     ::Optimizer,
     ::Type{MOI.VariableIndex},

src/main_algorithm.jl

@@ -97,10 +97,6 @@ function load!(m::Optimizer)
     # Populate data to create the bounding MIP model
     Alp.recategorize_var(m)             # Initial round of variable re-categorization
 
-    :Int in m.var_type_orig && error(
-        "Alpine does not support MINLPs with generic integer (non-binary) variables yet!",
-    )
-
     # Solver-dependent detection
     Alp._fetch_mip_solver_identifier(m)
     Alp._fetch_nlp_solver_identifier(m)

test/test_algorithm.jl

@@ -1052,5 +1052,9 @@ end
         "minlp_solver" => JUNIPER,
     )
     m = milp(solver = test_solver)
-    @test_throws "Alpine does not support MINLPs with generic integer (non-binary) variables yet!" JuMP.optimize!(m)
+    JuMP.optimize!(m)
+
+    @test termination_status(m) == MOI.OPTIMAL
+    @test isapprox(objective_value(m), 0; atol = 1e-4)
+    @test MOI.get(m, Alpine.NumberOfIterations()) == 0
 end