Some operations do not ignore trailing dimensions equal to 1

[mateuszbaran]

As in the title. Here is a small example:
addition in Base Julia arrays ignores trailing dimensions equal to 1:

julia> [1 2][:,1:1] + [1]
1×1 Array{Int64,2}:
 2

but in StaticArrays it does not:

julia> [1 2][:,1:1] +  SA[1]
ERROR: DimensionMismatch("Sizes ((1, 1), Size(1,)) of input arrays do not match")
Stacktrace:
 [1] _throw_size_mismatch(::Array{Int64,2}, ::Vararg{Any,N} where N) at /home/mateusz/.julia/dev/StaticArrays/src/traits.jl:198
 [2] same_size at /home/mateusz/.julia/dev/StaticArrays/src/traits.jl:192 [inlined]
 [3] macro expansion at /home/mateusz/.julia/dev/StaticArrays/src/mapreduce.jl:74 [inlined]
 [4] _map at /home/mateusz/.julia/dev/StaticArrays/src/mapreduce.jl:41 [inlined]
 [5] map at /home/mateusz/.julia/dev/StaticArrays/src/mapreduce.jl:34 [inlined]
 [6] +(::Array{Int64,2}, ::SArray{Tuple{1},Int64,1,1}) at /home/mateusz/.julia/dev/StaticArrays/src/linalg.jl:12
 [7] top-level scope at none:0

Do you think StaticArrays should have same_size modified to follow the behavior of Base?

[c42f]

I would expect this to work for broadcast, but for vector addition it's a bit of an oddity to allow this shape mismatch.

Having said that, I generally think we should follow Base so I guess we could relax our same_size a bit? I'm not sure whether it would have other consequences elsewhere.

[mateuszbaran]

Julia Base is a bit like that about trailing dimensions equal to 1. You can even do things like [2][1,1,1,1] or fill(2, 1, 2, 1, 1)[1, 1] and it works.

It shouldn't have many undesirable consequences. The main problem I can see is modifying things here to precisely match what Base is doing, specifically the number of axes of the resulting array. This may be quite complex.

[andyferris]

My personal opinion is Base is too permissive.

That said if this can be implemented neatly and removing runtime overhead I wouldn't stand in the way (since I'm usually the one harping on about following Base...).

---

Example from #788

julia> (SA[1,2,3])[1,1]
1

julia> (SA[1,2,3])[:,1]
ERROR: BoundsError: attempt to access ()
  at index [1]
Stacktrace:
 [1] getindex(::Tuple, ::Int64) at /Applications/Julia-1.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?
 [2] map at ./tuple.jl:180 [inlined] (repeats 2 times)
 [3] index_sizes at /Users/me/.julia/packages/StaticArrays/mlIi1/src/indexing.jl:80 [inlined]
 [4] getindex(::SArray{Tuple{3},Int64,1,3}, ::Colon, ::Int64) at /Users/me/.julia/packages/StaticArrays/mlIi1/src/indexing.jl:219
 [5] top-level scope at REPL[28]:1

julia> (SA[1,2,3])[1,:]
ERROR: BoundsError: attempt to access ()
  at index [1]
Stacktrace:
 [1] getindex(::Tuple, ::Int64) at /Applications/Julia-1.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?
 [2] map at ./tuple.jl:180 [inlined] (repeats 2 times)
 [3] index_sizes at /Users/me/.julia/packages/StaticArrays/mlIi1/src/indexing.jl:80 [inlined]
 [4] getindex(::SArray{Tuple{3},Int64,1,3}, ::Int64, ::Colon) at /Users/me/.julia/packages/StaticArrays/mlIi1/src/indexing.jl:219
 [5] top-level scope at REPL[27]:1