mapslices(f, d; dims=n) is type unstable

[lassepe]

A call to mapslices that takes a dims argument is type unstable. This is the case in all recent versions of Julia (1.1.1, 1.2.0, 1.3.1, 1.4.2).

For example:

d = rand(10)
mapslices(identity, d; dims=1)

Shows the following with @code_warntype:

Variables
  #unused#::Core.Compiler.Const(getfield(Base, Symbol("#kw##mapslices"))(), false)
  @_2::NamedTuple{(:dims,),Tuple{Int64}}
  @_3::Core.Compiler.Const(mapslices, false)
  f::Core.Compiler.Const(identity, false)
  A::Array{Float64,1}
  dims::Int64
  @_7::Int64

Body::Any
1 ─ %1  = Base.haskey(@_2, :dims)::Core.Compiler.Const(true, false)
│         %1
│         (@_7 = Base.getindex(@_2, :dims))
└──       goto #3
2 ─       Core.Compiler.Const(:(Core.UndefKeywordError(:dims)), false)
└──       Core.Compiler.Const(:(@_7 = Core.throw(%5)), false)
3 ┄       (dims = @_7)
│   %8  = (:dims,)::Core.Compiler.Const((:dims,), false)
│   %9  = Core.apply_type(Core.NamedTuple, %8)::Core.Compiler.Const(NamedTuple{(:dims,),T} where T<:Tuple, false)
│   %10 = Base.structdiff(@_2, %9)::Core.Compiler.Const(NamedTuple(), false)
│   %11 = Base.pairs(%10)::Core.Compiler.Const(Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}(), false)
│   %12 = Base.isempty(%11)::Core.Compiler.Const(true, false)
│         %12
└──       goto #5
4 ─       Core.Compiler.Const(:(Base.kwerr(@_2, @_3, f, A)), false)
5 ┄ %16 = Base.:(#mapslices#111)(dims, @_3, f, A)::Any
└──       return %16

Due to this type instability, things like Statistics.median(d; dims=n) are also type unstable.

[timholy]

Essentially a duplicate of #26868