Type inference for constructor with tuple-of-Int parameter

[timholy]

julia> immutable MyType{SZ}
           data::Int
       end

julia> mt = MyType{(3,5)}(1)
MyType{(3,5)}(1)

julia> doubleall(mt::MyType{(3,5)}) = MyType{(6,10)}(2*mt.data)
doubleall (generic function with 1 method)

julia> typeof(doubleall(mt))
MyType{(6,10)} (constructor with 1 method)

julia> Base.return_types(doubleall, (typeof(mt),))
1-element Array{Any,1}:
 MyType{SZ}

The SZ is the surprising part; why isn't it MyType{(6,10)}?

This is relevant for the more interesting example in #7568, where

julia> Base.return_types(*, (typeof(AfI), typeof(BfI)))
1-element Array{Any,1}:
 FixedArrayI{Float64,2,SZ,6}

[JeffBezanson]

dup of #7060