proposal: allow conversion of bools to numerics.

[docmerlin]

A lot of branch-less algorithms are developed for c where a<b is 0 when false and 1 when true.

It would be really helpful to be able to use those algorithms by being able to have uint64(a<b) and it be uint64(0) or uint64(1) depending on a and b.

Note: this isn't an April fools proposal. (someone pointed out I should say this).

[seankhliao]

Duplicate of #7657 #9367 #45320

[ianlancetaylor]

It's important to not confuse an algorithm that is branchless in the Go code and an algorithm that is branchless in the generated machine code.

For example, when using the gccgo compiler, this Go function

func Cmp(a, b int) int {
	if a < b {
		return 0
	}
	return 1
}

compiles to this assembly code (ignoring the stack-splitting code):

	xorl	%eax, %eax
	cmpq	%rsi, %rdi
	setge	%al
	ret

[FiloSottile]

Larger branches will not be made branchless by the compiler, and numeric booleans are usually used with subtle.ConstantTimeSelect. This is why the crypto/subtle and filippo.io/edwards25519 return 0 or 1 instead of false or true.