reduce(max, ::Vector{Missing}) only works for certain lengths

[cstjean]

In julia 1.0.3,

julia> reduce(max, fill(missing, 15))    # good
missing

julia> reduce(max, fill(missing, 16))
ERROR: TypeError: non-boolean (Missing) used in boolean context
Stacktrace:
 [1] mapreduce_impl at ./reduce.jl:460 [inlined]
 [2] _mapreduce(::typeof(identity), ::typeof(max), ::IndexLinear, ::Array{Missing,1}) at ./reduce.jl:318
 [3] _mapreduce_dim(::Function, ::Function, ::NamedTuple{(),Tuple{}}, ::Array{Missing,1}, ::Colon) at ./reducedim.jl:305
 [4] #mapreduce#536 at ./reducedim.jl:301 [inlined]
 [5] mapreduce at ./reducedim.jl:301 [inlined]
 [6] #reduce#537 at ./reducedim.jl:345 [inlined]
 [7] reduce(::Function, ::Array{Missing,1}) at ./reducedim.jl:345
 [8] top-level scope at none:0

julia> reduce((a,b)->max(a,b), fill(missing, 16))
missing

[nalimilan]

On master:

julia> reduce(max, fill(missing, 16))
missing

Unfortunately, it's not fixed on 1.1. Maybe the fix can be backported though (if we can identify it).

[nalimilan]

Wait, the threshold has just increased to 257:

julia> reduce(max, fill(missing, 257))
ERROR: TypeError: non-boolean (Missing) used in boolean context
Stacktrace:
 [1] mapreduce_impl(::typeof(identity), ::typeof(max), ::Array{Missing,1}, ::Int64, ::Int64) at ./reduce.jl:485
 [2] _mapreduce(::typeof(identity), ::typeof(max), ::IndexLinear, ::Array{Missing,1}) at ./reduce.jl:320
 [3] _mapreduce_dim(::Function, ::Function, ::NamedTuple{(),Tuple{}}, ::Array{Missing,1}, ::Colon) at ./reducedim.jl:308
 [4] #mapreduce#543 at ./reducedim.jl:304 [inlined]
 [5] mapreduce at ./reducedim.jl:304 [inlined]
 [6] #reduce#544 at ./reducedim.jl:348 [inlined]
 [7] reduce(::Function, ::Array{Missing,1}) at ./reducedim.jl:348
 [8] top-level scope at REPL[11]:1

A more direct reproducer is Base.mapreduce_impl(identity, max, fill(missing, 257), 1, 257). We just need to replace v == v in that function with something like (ismissing(v) || v == v) or (v == v) !== false. Or it could be clearer to move that condition inside the loop and write !ismissing(v) && v != v && continue.

[KristofferC]

#35989