fatal error in type inference (type bound)

[mcabbott]

I got the above error locally with #40996, which I have never seen before.

CI sees it too, here: https://build.julialang.org/#/builders/7/builds/5932/steps/5/logs/stdio (17f851b)

arrayops                           (5) |         failed at 2021-11-13T02:50:50.671
Error During Test at /buildworker/worker/tester_linuxaarch64/build/share/julia/test/arrayops.jl:1149
  Got exception outside of a @test
  fatal error in type inference (type bound)
  Stacktrace:
    [1] mapslices(f::Main.Test99Main_arrayops.var"#18#31", A::Matrix{Int64}; dims::Int64)
      @ Base ./abstractarray.jl:2803
    [2] macro expansion
      @ /buildworker/worker/tester_linuxaarch64/build/share/julia/test/arrayops.jl:1196 [inlined]

[LilithHafner]

Reproducers (first two from @mcabbott):

julia> mapslices(tuple, [1 2; 3 4], dims=1)
1×2 Matrix{Tuple{Vector{Int64}}}:
 ([1, 3],)  ([2, 4],)

julia> mapslices(x -> tuple(x), [1 2; 3 4], dims=1)
ERROR: fatal error in type inference (type bound)
Stacktrace:
 [1] mapslices(f::var"#5#6", A::Matrix{Int64}; dims::Int64)
   @ Base ./abstractarray.jl:2896
 [2] top-level scope
   @ REPL[13]:1

julia> mapslices(argmax, rand(10,10); dims=2)
ERROR: fatal error in type inference (type bound)
Stacktrace:
 [1] mapslices(f::typeof(argmax), A::Matrix{Float64}; dims::Int64)
   @ Base ./abstractarray.jl:2896
 [2] top-level scope
   @ REPL[14]:1

[mcabbott]

One more example, in which ndims seems to change the regime?

julia> mapslices(findmin, rand(3); dims=1)  # works in 1D, and infers
1-element Vector{Tuple{Float64, Int64}}:
 (0.10724568875730223, 3)

julia> @code_warntype mapslices(findmin, rand(3); dims=1)
...
Body::Vector{Tuple{Float64, Int64}}

julia> mapslices(findmin, rand(3,3); dims=1)  # errors in 2D
ERROR: fatal error in type inference (type bound)

julia> mapslices(findmin, rand(3,3,3); dims=1) |> summary  # works in 3D, but does not infer
"1×3×3 Array{Tuple{Float64, Int64}, 3}"

julia> @code_warntype mapslices(findmin, rand(3,3,3); dims=1)
...
Body::Any