Creating SVector of a single SVector{1, Float64}

New to Julia
,
3

Thank you @longemen3000, SA constructor does create nested SVectors even when both have Size 1 (i.e. N=1 and Dim=1). But I would still like to understand what prevents the constructor with explicit type parameters (I have also tried directly using SArray constructor, without using the SVector alias, and got the same error). I checked the source code of StaticArraysCore.jl but couldn’t understand why:
A) SVector of a Size-2 Svector (or Size-2 SVector of Size-1 SVectors) either (1) uses the first inner construct of SArray, or (2) passes the convert_tuple,
B) while a SVector of a Size-1 Svector fails in the convert_tuple.

Moreover, SA constructor seems to be much slower when passed an existing SVector and makes an extra allocation, compared to passing directly the type parameters of the inner SVector. See a similar discussion on creating SVector of SVectors

julia> my_vector_2d
2-element SVector{2, Float64} with indices SOneTo(2):
 0.8
 0.9

julia> @benchmark SA[my_vector_2d]
BenchmarkTools.Trial: 10000 samples with 590 evaluations.
 Range (min … max):  197.217 ns …   6.917 μs  ┊ GC (min … max): 0.00% … 96.72%
 Time  (median):     209.234 ns               ┊ GC (median):    0.00%
 Time  (mean ± σ):   230.900 ns ± 153.116 ns  ┊ GC (mean ± σ):  1.39% ±  2.16%

  ▅▇█▇▃  ▂▂▁ ▁▁▂                                                ▁
  █████▇████▇██████████▇▇▆█▇███▇▇▇▆▆▆▆▆▆▆▆▇▆▆▆▆▅▆▆▅▆▅▅▆▅▅▅▆▅▅▅▅ █
  197 ns        Histogram: log(frequency) by time        486 ns <

 Memory estimate: 64 bytes, allocs estimate: 2.

julia> @benchmark SVector{1, SVector{2, Float64}}(my_vector_2d)
BenchmarkTools.Trial: 10000 samples with 942 evaluations.
 Range (min … max):  79.872 ns …  4.579 μs  ┊ GC (min … max): 0.00% … 98.13%
 Time  (median):     82.801 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   93.011 ns ± 85.076 ns  ┊ GC (mean ± σ):  1.75% ±  1.95%

  ▇█▇▂    ▁▂▁▂▃      ▁▁ ▁                                     ▁
  ████▇▇▇▆█████▆▅▆▇████▇████▇▇▇██▆▆▆▅▆▆▇▇▇▆▆▅▅▅▅▆▅▆▆▅▆▅▅▄▄▄▅▅ █
  79.9 ns      Histogram: log(frequency) by time       201 ns <

 Memory estimate: 32 bytes, allocs estimate: 1.

and here is an MWE without benchmarks:

using BenchmarkTools, StaticArrays

# Inner SVectors
my_vector_1d = SVector{1, Float64}(0.8)
my_vector_2d = SVector{2, Float64}(0.8, 0.9)

# Container SVectors
# Size-1 SVector of Size-1 SVector
# my_container_faulty = SVector{1, SVector{1, Float64}}(my_vector_1d)  # gives error
my_container_a = SA[my_vector_1d]
# Size-2 SVector of Size-1 SVectors
my_container_b = SVector{2, SVector{1, Float64}}(my_vector_1d, my_other_vector_1d)
# Size-1 SVector of Size-2 Svector
my_container_c = SVector{1, SVector{2, Float64}}(my_vector_2d)