Merge pull request #382 from JuliaOpt/bl/voidnothing

Fix v0.7 warning for Void -> Nothing

Author: blegat

src/Utilities/Utilities.jl

@@ -1,6 +1,6 @@
 module Utilities
 
-using Compat # For lastindex
+using Compat # For lastindex and Nothing
 
 using MathOptInterface
 const MOI = MathOptInterface

src/Utilities/cachingoptimizer.jl

@@ -24,7 +24,7 @@ A `CachingOptimizer` has two modes of operation (`CachingOptimizerMode`):
 """
 mutable struct CachingOptimizer{MT<:MOI.ModelLike} <: MOI.AbstractOptimizer
     model_cache::MT
-    optimizer::Union{Void,MOI.AbstractOptimizer}
+    optimizer::Union{Nothing,MOI.AbstractOptimizer}
     state::CachingOptimizerState
     mode::CachingOptimizerMode
     model_to_optimizer_map::IndexMap