julia> struct A{T}
           x::Vector{T}
       end

julia> A(rand(1))
A{Float64}([0.7610381409441728])

julia> A{Float64}(rand(1))
A{Float64}([0.5092874009390502])

julia> using ConcreteStructs

julia> @concrete struct B{T}
           x <: Vector{T}
       end

julia> B(rand(1))
ERROR: MethodError: no method matching B(::Vector{Float64})
The type `B` exists, but no method is defined for this combination of argument types when trying to construct it.
Stacktrace:
 [1] top-level scope
   @ REPL[6]:1

julia> B{Float64}(rand(1))
B{Float64, Vector{Float64}}([0.038474710287414005])

julia> @concrete struct C{T}
           x::Vector{T}
       end

julia> C(rand(1))
C{Float64}([0.41418023308916496])

julia> C{Float64}(rand(1))
C{Float64}([0.7672194655197099])