Do not promote to Any

Before:

    julia> push!!([Init(+)], 0)
    2-element Array{Any,1}:
      Init(+)
     0

After:

    julia> push!!([Init(+)], 0)
    2-element Array{Union{InitialValues.InitialValueOf{typeof(+)}, Int64},1}:
      Init(+)
     0

src/InitialValues.jl

@@ -203,6 +203,8 @@ end
 @disambiguate Base.min Missing
 @disambiguate Base.max Missing
 
+Base.promote_rule(::Type{I}, ::Type{S}) where {I<:InitialValue,S} = Union{I,S}
+
 const ZeroType = Union{
     SpecificInitialValue{typeof(+)},
     SpecificInitialValue{typeof(Base.add_sum)},

test/interop/test_bangbang.jl

@@ -12,4 +12,9 @@ using InitialValues: Init, hasinitialvalue
     @test append!!([1], Init(append!!)) == [1]
 end
 
+@testset "promote" begin
+    init = Init(+)
+    @test eltype(push!!([init], 0)) == Union{typeof(init),Int}
+end
+
 end  # module

test/test_basics.jl

@@ -4,8 +4,9 @@ using Test
 using InitialValues
 using InitialValues: isknown, hasinitialvalue
 
+OPS = [*, +, |, &, min, max, Base.add_sum, Base.mul_prod]
 
-@testset for op in [*, +, |, &, min, max, Base.add_sum, Base.mul_prod]
+@testset for op in OPS
     @test op(Init(op), :anything) === :anything
     @test hasinitialvalue(op)
     @test hasinitialvalue(typeof(op))
@@ -39,6 +40,13 @@ end
     @test max(Init(max), missing) === missing
 end
 
+@testset "promote" begin
+    for op in OPS
+        T = typeof(Init(op))
+        @test promote_type(T, Val{0}) == Union{T,Val{0}}
+    end
+end
+
 @testset "convert" begin
     @testset "float" begin
         @test float(Init(+)) === 0.0