Compatibility for overriding `cld`/`fld`

[sostock]

The implementation on current master (see below) claims to provide compatibility for overriding cld/fld, but actually doesn’t (at least in the case of HalfIntegers.jl, which is currently broken on master for this reason).

julia/base/div.jl

Lines 223 to 233 in 8707744

	# These are kept for compatibility with external packages overriding fld/cld.
	# In 2.0, packages should extend div(a,b,r) instead, in which case, these can
	# be removed.
	fld(x::Real, y::Real) = div(promote(x,y)..., RoundDown)
	cld(x::Real, y::Real) = div(promote(x,y)..., RoundUp)
	fld(x::Signed, y::Unsigned) = div(x, y, RoundDown)
	fld(x::Unsigned, y::Signed) = div(x, y, RoundDown)
	cld(x::Signed, y::Unsigned) = div(x, y, RoundUp)
	cld(x::Unsigned, y::Signed) = div(x, y, RoundUp)
	fld(x::T, y::T) where {T<:Real} = throw(MethodError(div, (x, y, RoundDown)))
	cld(x::T, y::T) where {T<:Real} = throw(MethodError(div, (x, y, RoundUp)))

HalfIntegers.jl defines fld(x::T, y::T) where T<:HalfInteger. Due to the changes on master, this method isn’t called anymore when fld is called with two different types of HalfIntegers. In order for the above code to fulfill its purpose, the first two and last two methods above could be replaced with

fld(x::Real, y::Real) = fld(promote(x,y)...)
cld(x::Real, y::Real) = cld(promote(x,y)...)
fld(x::T, y::T) where {T<:Real} = div(x, y, RoundDown)
cld(x::T, y::T) where {T<:Real} = div(x, y, RoundUp)

[Keno]

Yeah, this fallback doesn't work 100% of the time, but your suggestion doesn't work either, because it would cause dispatch loops due to the reverse fallbacks elsewhere in the code. This is the best I could come up with for the time being. Packages were quite inconsistent about how these methods were being overloaded. I'd recommend additionally defining fld(x::HalfInteger, y::HalfInteger) and the three-argument div method in your package.

[sostock]

Ok, if compatibility cannot be achieved for all packages, I will update my package and mark old versions as incompatible with Julia 1.4.