fix bug with max_values in union! (JuliaLang#30315)

base/abstractset.jl

@@ -69,9 +69,11 @@ function union!(s::AbstractSet, sets...)
 end
 
 max_values(::Type) = typemax(Int)
-max_values(T::Type{<:Union{Nothing,BitIntegerSmall}}) = 1 << (8*sizeof(T))
-max_values(T::Union) = max(max_values(T.a), max_values(T.b))
+max_values(T::Union{map(X -> Type{X}, BitIntegerSmall_types)...}) = 1 << (8*sizeof(T))
+# saturated addition to prevent overflow with typemax(Int)
+max_values(T::Union) = max(max_values(T.a), max_values(T.b), max_values(T.a) + max_values(T.b))
 max_values(::Type{Bool}) = 2
+max_values(::Type{Nothing}) = 1
 
 function union!(s::AbstractSet{T}, itr) where T
  haslength(itr) && sizehint!(s, length(s) + length(itr))

test/sets.jl

@@ -628,6 +628,27 @@ end
  end
 end
 
+@testset "optimized union! with max_values" begin
+ # issue #30315
+ T = Union{Nothing, Bool}
+ @test Base.max_values(T) == 3
+ d = Set{T}()
+ union!(d, (nothing, true, false))
+ @test length(d) == 3
+ @test d == Set((nothing, true, false))
+ @test nothing in d
+ @test true in d
+ @test false in d
+
+ for X = (Int8, Int16, Int32, Int64)
+ @test Base.max_values(Union{Nothing, X}) == (sizeof(X) < sizeof(Int) ?
+ 2^(8*sizeof(X)) + 1 :
+ typemax(Int))
+ end
+ # this does not account for non-empty intersections of the unioned types
+ @test Base.max_values(Union{Int8,Int16}) == 2^8 + 2^16
+end
+
 struct OpenInterval{T}
  lower::T
  upper::T