Fix CleverDict with negative indices (#1168)

Author: blegat

src/Utilities/CleverDicts.jl

@@ -167,8 +167,12 @@ end
 
 function Base.setindex!(c::CleverDict{K, V}, value::V, key::K)::V where {K, V}
     h = c.hash(key)::Int64
-    if h > c.last_index
-        c.last_index = ifelse(h == c.last_index + 1, h, -1)
+    if c.last_index != -1
+        if h == c.last_index + 1
+            c.last_index = h
+        elseif h <= 0 || h > c.last_index
+            c.last_index = -1
+        end
     end
     if 1 <= h <= length(c.vector) && _is_dense(c)
         c.vector[h] = value

test/Utilities/CleverDicts.jl

@@ -238,4 +238,15 @@ CleverDicts.index_to_key(::Type{MyKey}, index::Int64) = MyKey(index)
         @test d[4] == 0.25
         @test d[6] == 0.75
     end
+
+    @testset "Negative index" begin
+        d = CleverDicts.CleverDict{MathOptInterface.VariableIndex, String}()
+        d[MathOptInterface.VariableIndex(-3)] = "a"
+        @test d[MathOptInterface.VariableIndex(-3)] == "a"
+        @test_throws ErrorException CleverDicts.add_item(d, "b")
+        d[MathOptInterface.VariableIndex(0)] = "b"
+        @test d[MathOptInterface.VariableIndex(-3)] == "a"
+        @test d[MathOptInterface.VariableIndex(0)] == "b"
+        @test_throws ErrorException CleverDicts.add_item(d, "c")
+    end
 end