This feels like it's too specific. What if a type is known to be too large for a Float64? With float it's at least possible to hook into the logic and add an appropriate method giving the appropriate type. With Float64 here that's no longer possible.

That is true -- except that using float and giving a floating point type larger than Float64 is was caused the stack to overflow as the result of float was a floating point value larger than Float64 which dispatched ini_dec(::Real) over and over. Do you know what this is intended to do with e.g. Double64 because float(x::Double64) === x as fits the definition of float. Indeed
float(x::T) where {T <: AbstractFloat} = x and ini_dec, fix_dec do not provide workable fallbacks then. [converting to BigFloat and passing those values did not work either -- dunno why, as BigFloats per se seem to be handled]

I am open to a better approach. I guess step 1 is understanding those two functions and why they exist and how they are used -- this is the only source file in which they appear.

Yeah, when you get down to it, you have to see what the underlying algorithms support; over at #32859, I actually changed it from float to Float64, because realistically the Ryu algorithm expects that, Float32, or Float16 explicitly. Could we have a good generic fallback that did a slightly slower thing (using BigInt/BigFloat?)? Probably. But switching this to Float64 better reflects what our current fallbacks need.

well said. given the underlying algorithmic implicit type constraints, rather than float(x) [which is the identity operation for any ::AbstractFloat and many ::Real types], the function required is fastfloat(x) --- unless rewriting that part underlying Printf is to be done. fastfloat(x) is x for IEEEFloats and Float64(x) otherwise. (I do not see the advantage in this use to have the result be Float32(x) for [U]Int8 and [U]Int16 values. If introducing this function for export, then that refinement makes sense.)

@StefanKarpinski <oops ... no functional air conditioning for days ... nvr mnd>

Made unnecessary by #32859