Using parametric type inside outer parametric constructor

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My mistake, I copied the code and forgot to change the commented lines.

It seems clear to me that what I pointed in your original MWE cannot be exactly same that affected OP’s MWE. In OP’s MWE the problem happens because somehow in the first function called T is not recognized within the body of the first function called.

@schneiderfelipe Than for your kind words, but while I think I got it right for @lmiq MWE, I do not think I solved your original problem.

Now… this is funny:

julia> struct Foo{T<:Real, S<:AbstractMatrix{<:T}}
               data::S
               n::Int
       
               function Foo{T, S}(data, n) where {T<:Real, S<:AbstractMatrix{<:T}}
                   println("Foo{T, S}(data, n)")
                   Base.require_one_based_indexing(data)
                   new{T, S}(data, n)
               end
       end
       
       function Foo(A::AbstractMatrix{T}, n::Int) where {T<:Real}
           println("Foo(A, n)")
           return Foo{T, typeof(A)}(A, n)
       end
       
       # Changed the signature below to take both T and S even if S is not used
       function Foo{T, S}(::UndefInitializer, n::Int) where {T<:Real, S <: AbstractMatrix{<:T}}
           println("Foo{T, S}(undef, n)")
           d = fld(n * (n + 1), 2)
       
           # The next two lines should substitute each other,
           # but the commented one throws an error.
       
           A = Matrix{T}(undef, d, d)  # UndefVarError: T not defined
           # A = zeros(d, d)
       
           return Foo(A, n)
       end

julia> Foo{Float64, Matrix{Float64}}(undef, 2)
Foo{T, S}(undef, n)
Foo(A, n)
Foo{T, S}(data, n)
Foo{Float64,Array{Float64,2}}([0.0 0.0 0.0; 0.0 0.0 0.0; 0.0 0.0 0.0], 2)

Why? Why the hell?

My guess now is that, maybe, just maybe, we are not allowed to define an outer constructor taking less type parameters than the struct itself?

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