Throw UnsupportedAttribute when mip starts not supported. (#76)

* Throw UnsupportedAttribute when mip starts not supported.

* Update docstring

src/solver_interface.jl

@@ -603,8 +603,17 @@ Delete the columns `start_col`, `start_col+1`, ..., `end_col` from the model `m`
 function delete_variables! end
 
 """
-    add_mip_starts!(m, cols::Vector{Int}, x::Vector{Float64})::Nothing
+    add_mip_starts!(model::M, cols::Vector{Int}, x::Vector{Float64})::Nothing
 
-Add the MIP start `x` for the variables in the columns `cols` of the model `m`.
+Add a primal start `x` for the variables in the columns `cols` of `model`.
+
+Note that if this method is implemented, solvers of type `M` must also declare
+that they support VariablePrimalStarts by overloading the following method:
+
+    function MOI.supports(model::M,
+                          ::MOI.VariablePrimalStart,
+                          ::Type{MOI.VariableIndex})
+        return true
+    end
 """
 function add_mip_starts! end

src/variables.jl

@@ -200,12 +200,16 @@ end
 #=
     MIP starts
 =#
-function MOI.supports(::LinQuadOptimizer, ::MOI.VariablePrimalStart, ::Type{VarInd})
+function MOI.supports(
+        ::LinQuadOptimizer, ::MOI.VariablePrimalStart, ::Type{VarInd})
     return false
 end
 
 function MOI.set(model::LinQuadOptimizer, ::MOI.VariablePrimalStart,
                   indices::Vector{VarInd}, values::Vector{Float64})
+    if !MOI.supports(model, MOI.VariablePrimalStart(), MOI.VariableIndex)
+        throw(MOI.UnsupportedAttribute(MOI.VariablePrimalStart()))
+    end
     add_mip_starts!(model, get_column.(Ref(model), indices), values)
 end
 

test/runtests.jl

@@ -451,3 +451,9 @@ end
         @test model.inner.vartype[1] == Cchar('C')
     end
 end
+
+@testset "VariablePrimalStart" begin
+    model = LQOI.MockLinQuadOptimizer()
+    x = MOI.add_variable(model)
+    @test_throws MOI.UnsupportedAttribute MOI.set(model, MOI.VariablePrimalStart(), x, 1.0)
+end