Julia: Inexact error when trying to get the integer part of a BigFloat - type-conversion

Question

I am interested in getting the digits of a BigFloat in the form of bytes. I get a very strange error that I cannot debug. I provide a minimal example where the error appears.
function floatToBytes(x::BigFloat)
ret = zeros(UInt8, 4)
xs = significand(x)/2
b = UInt8(0)
for i = 1:4
xs *= 256
b = trunc(UInt8, xs)
ret[i] = b
xs -= b
end
return ret
end
println( floatToBytes(BigFloat(0.9921875001164153)) )
println( floatToBytes(BigFloat(0.9960937501164153)) )
What I get when running this is
UInt8[0xfe, 0x00, 0x00, 0x00]
ERROR: LoadError: InexactError()
Stacktrace:
[1] trunc(::Type{UInt8}, ::BigFloat) at ./mpfr.jl:201
etc.
It seems that it doesn't want to turn 255 into a UInt8. I can circumvent the problem by defining the function as
function floatToBytes(x::BigFloat)
ret = zeros(UInt8, 4)
xs = significand(x)/2
b = UInt8(0)
for i = 1:4
xs *= 256
try
b = trunc(UInt8, xs)
catch
b = trunc(UInt8, xs-1)+UInt8(1)
end
ret[i] = b
xs -= b
end
return ret
end
But this is highly unsatisfactory. What is going on here?

Answer 1

The problem looks like a bug in trunc for BigFloat. The problem is the current code does (typemin(T) <= x <= typemax(T)) || throw(InexactError(:trunc, T, x)) which throws an error because x is larger than 255 which is the typemax.
It actually needs to do the trunc in BigFloat domain and then cast to T (and have the cast check for typemax).
I've opened an issue regarding this at: https://github.com/JuliaLang/julia/issues/24041
In the meantime, a solution could be to do:
UInt8(trunc(xs))
i.e. trunc first and cast later. For example:
julia> UInt8(trunc(BigFloat(0.9960937501164153)*256))
0xff