Consider the type with the promoting constructor

```
struct Foo{T,TX<:AbstractVector{T},TFX}
Δ::T
x::TX
fx::TFX
residual_norm::T
converged::Bool
iterations::Int
end
function Foo(Δ::T1, x::AbstractVector{T2}, fx, residual_norm::T3, converged,
iterations) where {T1 <: Real, T2 <: Real, T3 <: Real}
T = promote_type(T1, T2, T3)
Foo(T(Δ), T.(x), fx, T(residual_norm), converged, iterations)
end
```

but

```
julia> Foo(1, [1.0, 2.0], nothing, 1.0, true, 0)
ERROR: MethodError: Foo(::Float64, ::Array{Float64,1}, ::Nothing, ::Float64, ::Bool, ::Int64) is ambiguous. Candidates:
(::Type{Foo})(Δ::T, x::TX, fx::TFX, residual_norm::T, converged::Bool, iterations::Int64) where {T, TX<:AbstractArray{T,1}, TFX} in Main at REPL[1]:2
(::Type{Foo})(Δ::T1, x::AbstractArray{T2,1}, fx, residual_norm::T3, converged, iterations) where {T1<:Real, T2<:Real, T3<:Real} in Main at REPL[2]:3
Possible fix, define
Foo(::T1, ::Union{TX, TX}, ::TFX, ::Union{T3, T}, ::Bool, ::Int64) where {T1<:Real, T2<:Real, T3<:T2, TX<:AbstractArray{T2,1}, T<:Real, TX<:AbstractArray{T,1}, TFX}
Stacktrace:
[1] Foo(::Int64, ::Array{Float64,1}, ::Nothing, ::Float64, ::Bool, ::Int64) at ./REPL[2]:4
[2] top-level scope at REPL[3]:1
```

The part I don’t get is

- why doesn’t this work,
- why the
`::Union{TX, TX}`

, - why the repeated
`TX`

type parameter.

Is this a bug? Using 1.4-rc1, but could reproduce on 1.3.