Summing multiple dimensions of a Static Array with `sum()` throws an error

[mroavi]

Minimal example:

julia> using StaticArrays

julia> a = @SArray rand(2,2,2);

julia> sum(a, dims=(1,2))
ERROR: MethodError: no method matching _mapfoldl(::typeof(identity), ::typeof(+), ::Tuple{Int64, Int64}, ::StaticArrays._InitialValue, ::Size{(2, 2, 2)}, ::SArray{Tuple{2, 2, 2}, Float64, 3, 8})
Closest candidates are:
  _mapfoldl(::Any, ::Any, ::Colon, ::Any, ::Size{S}, ::StaticArray...) where S at /home/mroavi/.julia/packages/StaticArrays/LJQEe/src/mapreduce.jl:115
  _mapfoldl(::Any, ::Any, ::Val{D}, ::Any, ::Size{S}, ::StaticArray) where {S, D} at /home/mroavi/.julia/packages/StaticArrays/LJQEe/src/mapreduce.jl:161
Stacktrace:
 [1] _mapreduce
   @ ~/.julia/packages/StaticArrays/LJQEe/src/mapreduce.jl:113 [inlined]
 [2] _reduce (repeats 2 times)
   @ ~/.julia/packages/StaticArrays/LJQEe/src/mapreduce.jl:209 [inlined]
 [3] #sum#285
   @ ~/.julia/packages/StaticArrays/LJQEe/src/mapreduce.jl:250 [inlined]
 [4] top-level scope
   @ REPL[7]:1

[mcabbott]

Even just one dimension:

julia> sum(SA[1 2; 3 4], dims=1)  # ok!
1×2 SMatrix{1, 2, Int64, 2} with indices SOneTo(1)×SOneTo(2):
 4  6

julia> sum(SA[1 2; 3 4], dims=(1,))  # same error
ERROR: MethodError: no method matching _mapfoldl(::typeof(identity), ::typeof(+), ::Tuple{Int64}, ::StaticArrays._InitialValue, ::Size{(2, 2)}, ::SMatrix{2, 2, Int64, 4})
...

julia> sum(SA[1 2; 3 4], dims=Val(1))  # ok!
1×2 SMatrix{1, 2, Int64, 2} with indices SOneTo(1)×SOneTo(2):
 4  6

julia> sum(SA[1 2; 3 4], dims=Val((1,)))  # different error
ERROR: MethodError: no method matching getindex(::Tuple{Int64, Int64}, ::Tuple{Int64})
Closest candidates are:
  getindex(::Tuple, ::Int64) at tuple.jl:29
  getindex(::Tuple, ::Real) at tuple.jl:30
  getindex(::Tuple, ::Colon) at tuple.jl:33
  ...
Stacktrace:
 [1] #s46#288
   @ ~/.julia/dev/StaticArrays/src/mapreduce.jl:175 [inlined]
 [2] var"#s46#288"(S::Any, D::Any, ::Any, f::Any, op::Any, dims::Any, init::Any, #unused#::Any, a::Any)
   @ StaticArrays ./none:0
 [3] (::Core.GeneratedFunctionStub)(::Any, ::Vararg{Any})
   @ Core ./boot.jl:576
 [4] _mapreduce
   @ ~/.julia/dev/StaticArrays/src/mapreduce.jl:113 [inlined]
 [5] _reduce (repeats 2 times)
   @ ~/.julia/dev/StaticArrays/src/mapreduce.jl:209 [inlined]
 [6] #sum#298
   @ ~/.julia/dev/StaticArrays/src/mapreduce.jl:250 [inlined]