EMPTY_OPTIMIZER mode after empty!

docs/src/submodules/Utilities/overview.md

@@ -199,6 +199,11 @@ with model cache MOIU.GenericModel{Float64,MOIU.ModelFunctionConstraints{Float64
 with optimizer MOIU.GenericOptimizer{Float64,MOIU.VectorOfConstraints{MOI.VectorAffineFunction{Float64},MOI.Complements}}
 ```
 
+!!! info
+    Calling `MOI.empty!(model)` also resets the state to `EMPTY_OPTIMIZER`.
+    So after emptying a model, the modification will only be applied to the
+    cache.
+
 Use [`Utilities.drop_optimizer`](@ref) to go from any state to `NO_OPTIMIZER`:
 ```jldoctest pathoptimizer
 julia> MOI.Utilities.drop_optimizer(model)

src/Utilities/cachingoptimizer.jl

@@ -270,6 +270,7 @@ function MOI.empty!(m::CachingOptimizer)
     MOI.empty!(m.model_cache)
     if m.state == ATTACHED_OPTIMIZER
         MOI.empty!(m.optimizer)
+        m.state = EMPTY_OPTIMIZER
     end
     m.model_to_optimizer_map = IndexMap()
     m.optimizer_to_model_map = IndexMap()

test/Utilities/cachingoptimizer.jl

@@ -349,6 +349,14 @@ end
     in mode MANUAL
     with model cache $(MOIU.Model{Float64})
     with optimizer nothing""")
+
+    MOI.empty!(s)
+    MOIU.reset_optimizer(m, s)
+    @test MOIU.state(m) == MOIU.EMPTY_OPTIMIZER
+    MOIU.attach_optimizer(m)
+    @test MOIU.state(m) == MOIU.ATTACHED_OPTIMIZER
+    MOI.empty!(m)
+    @test MOIU.state(m) == MOIU.EMPTY_OPTIMIZER
 end
 
 @testset "CachingOptimizer AUTOMATIC mode" begin
@@ -486,6 +494,9 @@ end
     in mode AUTOMATIC
     with model cache $(MOIU.Model{Float64})
     with optimizer $(MOIU.MockOptimizer{MOIU.Model{Float64}})""")
+
+    MOI.empty!(m)
+    @test MOIU.state(m) == MOIU.EMPTY_OPTIMIZER
 end
 
 @testset "Constructor with optimizer" begin