Missing round methods for RoundNearest

[iamed2]

A number of round methods which take a RoundingMode do not accept RoundNearest (but do accept the other modes). Evidence:

julia> round(0.5, RoundNearest)
ERROR: MethodError: no method matching round(::Type{Float64}, ::Float64, ::RoundingMode{:Nearest})
Closest candidates are:
  round{T}(::Type{T}, ::Rational{T<:Integer}, ::RoundingMode{:Nearest})
  round{T<:Integer}(::Type{T<:Integer}, ::AbstractFloat, ::RoundingMode{T})
  round{T,R}(::Type{T}, ::AbstractArray{R,1}, ::RoundingMode{T})
  ...
 in eval(::Module, ::Any) at ./boot.jl:267

julia> round(0.5, RoundDown)
0.0

This appears to be the case on 0.4 and master.

The missing methods are round{T<:FloatingPoint}(::T, ::RoundingMode{:Nearest}). There are other methods I found which depend on those, such as round(::Type{Int64}, ::Float64, ::RoundingMode{:Nearest}).

It seems to me like either round(::Number, RoundingMode{:Nearest}) should call round(::Number) or the reverse should be true. If the latter option is preferred, there's a bunch of code overhaul to do, and packages probably have to be updated as well.

[simonbyrne]

Yes, it should. Unfortunately, LLVM doesn't support it, though hopefully they should in future as roundeven is part of the most recent C floating point technical spec: see #8750 (comment) for the full details.

[iamed2]

Oh, so it functionally can't be done, and round doesn't actually do RoundNearest? If that's the case, why do the docs say that the default is RoundNearest?

[simonbyrne]

round(x) uses the current rounding mode (set via setrounding), which by default is RoundNearest.

[iamed2]

If I understand correctly, it is impossible to have a RoundNearest rounding mode for floats because there's no way to ensure the correct behaviour other than setting the rounding mode?

[simonbyrne]

Yes, exactly.

[kshyatt]

It seems like this was an issue with the modes LLVM provides, which we can't do much about. I'm going to close this - feel free to reopen if I'm wrong in doing so.

[iamed2]

I think it would be nice to have a living issue for it, but this isn't it.