Constructors for parametric types

General Usage
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4

Julia doesn’t enforce this. As you say, the error comes from the type itself, before the constructor call even occurs. The constructor’s signature cannot widen the type’s parameters.

julia> struct X{T<:Real}
         function X{T}() where T<:Number
           return "hello"
         end
       end

julia> X{Int}()
"hello"

julia> X{Int}
X{Int64}

julia> X{Complex}
ERROR: TypeError: in X, in T, expected T<:Real, got Type{Complex}
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