Cannot compute quantile(x::Array{Millisecond,1})

[ianfiske]

ms = Millisecond.(1:10)
quantile(ms, 0.5)

gives

ERROR: MethodError: no method matching isnan(::Millisecond)
Closest candidates are:
  isnan(::BigFloat) at mpfr.jl:881
  isnan(::Missing) at missing.jl:100
  isnan(::Float16) at float.jl:537
  ...
Stacktrace:
 [1] _quantilesort!(::Array{Millisecond,1}, ::Bool, ::Float64, ::Float64) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/Statistics/src/Statistics.jl:960
 [2] quantile!(::Array{Millisecond,1}, ::Float64; sorted::Bool, alpha::Float64, beta::Float64) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/Statistics/src/Statistics.jl:943
 [3] quantile(::Array{Millisecond,1}, ::Float64; sorted::Bool, alpha::Float64, beta::Float64) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/Statistics/src/Statistics.jl:1052
 [4] quantile(::Array{Millisecond,1}, ::Float64) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/Statistics/src/Statistics.jl:1052
 [5] top-level scope at REPL[14]:1
 [6] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1088

on Julia 1.5.2.

The root issue is of course

julia> isnan(Millisecond(1))
ERROR: MethodError: no method matching isnan(::Millisecond)
Closest candidates are:
  isnan(::BigFloat) at mpfr.jl:881
  isnan(::Missing) at missing.jl:100
  isnan(::Float16) at float.jl:537

since _quantilesort! calls isnan(x[end])

[ianfiske]

Would

isnan(::TimePeriod) = false

be an acceptable fix?

[mschauer]

Unitful defines isnan https://github.com/PainterQubits/Unitful.jl/blob/2be7e0692cafd948b66132920ee407b73220717f/src/quantities.jl
and base should perhaps define it for TimePeriods as well.

[nalimilan]

As noted at JuliaLang/julia#37748, I think we'd better check applicable or method_exists, as we don't want to define isnan on all types that quantile should support.