proposal: spec: add decimal float types (IEEE 754-2008)

[typeless]

I know there was a proposal about decimal as an std package.
But according to https://en.wikipedia.org/wiki/Decimal_floating_point, there have already been multiple compliant hardware implementations, and supported by several commonly used programming languages. I think having decimal as first-class citizen like the other part of the IEEE-754 standard makes sense.

The upside is we don't have to spend the effort on API design upfront to make it useful like math.big. We can just limit the scope of the operations only within the Go's existing arithmetic operators. Designing a good numerical/arithmetic API beyond the conventional operators is harder and can be deferred later with no harm.

[bradfitz]

Related: #12127, #19770

[bronze1man]

I think this proposal can be implement outside golang runtime, builtin and builtin package.
Does any implement exist?

[typeless]

@bronze1man Yes, there are thirty-party packages and proposals for including a decimal package into std. But this proposal is about adding fixed-size decimal floating point types, along with the existing binary floating point types using the existing arithmetic operators, rather than introducing a new package.

[deanveloper]

I made my own post for this without seeing this one first. Here are the reasons I believe that a decimal float type would be good for Go - #26699

[deanveloper]

Adding onto this - If we had operator overloading, this could be implemented as a 3rd party lib instead