struct Foo{T}
    x::T
end

makevec(range) = map(range) do i
    if i > 3
        Foo(Float64(i))
    else
        Foo(i)
    end
end

bar(vec::Vector{Foo}) = length(vec)

vec = makevec(1:5)
typeof(vec) <: Vector{Foo} # true

bar(vec)

vec = makevec(1:3)
typeof(vec) <: Vector{Foo} # false
typeof(vec) <: Vector{Foo{T}} where T # true

bar(vec) # error

struct Foo{T}
    x::T
end

makevec(range) = map(range) do i
    if i > 3
        Foo(Float64(i))
    else
        Foo(i)
    end
end

bar(vec::Vector{T}) where {T <: Foo} = length(vec)

v = makevec(1:5)

bar(v)

v = makevec(1:3)

bar(v)

vec = makevec(1:3)

struct Bar1
    vec::Vector{Foo}
end
Bar1(vec)

struct Bar2{T}
    x::T
    vec::Vector{Foo}
end
Bar2(1, vec) # error