Can't sample random Core.BFloat16

[Seelengrab]

Core.BFloat16 was added as a new AbstractFloat in #51470, but so far it doesn't seem possible to call rand(BFloat16):

julia> rand(Core.BFloat16)
ERROR: MethodError: no method matching Random.Sampler(::Type{TaskLocalRNG}, ::Random.SamplerTrivial{Random.CloseOpen01{…}, Core.BFloat16}, ::Val{1})
This error has been manually thrown, explicitly, so the method may exist but be intentionally marked as unimplemented.

Closest candidates are:
  Random.Sampler(::Type{<:AbstractRNG}, ::Random.Sampler, ::Union{Val{1}, Val{Inf}})
   @ Random ~/julia/usr/share/julia/stdlib/v1.12/Random/src/Random.jl:145
  Random.Sampler(::Type{<:AbstractRNG}, ::Any, ::Union{Val{1}, Val{Inf}})
   @ Random ~/julia/usr/share/julia/stdlib/v1.12/Random/src/Random.jl:181
  Random.Sampler(::Type{<:AbstractRNG}, ::BitSet, ::Union{Val{1}, Val{Inf}})
   @ Random ~/julia/usr/share/julia/stdlib/v1.12/Random/src/generation.jl:488
  ...

Stacktrace:
 [1] Random.Sampler(T::Type{TaskLocalRNG}, sp::Random.SamplerTrivial{Random.CloseOpen01{…}, Core.BFloat16}, r::Val{1})
   @ Random ~/julia/usr/share/julia/stdlib/v1.12/Random/src/Random.jl:145
 [2] Random.Sampler(rng::TaskLocalRNG, x::Random.SamplerTrivial{Random.CloseOpen01{Core.BFloat16}, Core.BFloat16}, r::Val{1})
   @ Random ~/julia/usr/share/julia/stdlib/v1.12/Random/src/Random.jl:139
 [3] rand(rng::TaskLocalRNG, X::Random.SamplerTrivial{Random.CloseOpen01{Core.BFloat16}, Core.BFloat16})
   @ Random ~/julia/usr/share/julia/stdlib/v1.12/Random/src/Random.jl:253
 [4] rand(rng::TaskLocalRNG, ::Type{Core.BFloat16})
   @ Random ~/julia/usr/share/julia/stdlib/v1.12/Random/src/Random.jl:256
 [5] rand(::Type{Core.BFloat16})
   @ Random ~/julia/usr/share/julia/stdlib/v1.12/Random/src/Random.jl:259
 [6] top-level scope
   @ REPL[31]:1
Some type information was truncated. Use `show(err)` to see complete types.

This was found in these nightly tests (CI log is here). I can mark that particular combination of arguments broken for now, but it would be cool to get this functionality at some point :)

I'm not 100% sure the label float16 is correct. This is about BFloat16 after all, which is slightly different. Feel free to adjust as necessary!

[gbaraldi]

This is a type that doesn't have a full implementation in base, it's there to make sure codegen understands it in a packaged like https://github.com/JuliaMath/BFloat16s.jl

[Seelengrab]

It's a bit odd to me to define a type in Base that claims to be AbstractFloat, without actually behaving like that at all and instead requiring an external package to access that functionality :/

[giordano]

I'm going to close this ticket as the random sampling method is implemented in BFloat16s.jl. It's true BFloat16s.jl is basically doing type piracy, but at least there's coordination with Base, it was expected to behave like that already in the above mentioned #51470, and if anything that's a separate issue.