Improving speed of runtime dispatch detector

General Usage Performance
22

Okay all is good, I fixed it here by moving back to the closure approach:

function _stable(fex::Expr)
    func = splitdef(fex)

    ...
    @gensym closure T
    func[:body] = quote
        let $closure() = $(func[:body]), $T = $(Base).promote_op($closure)
            if !$(Base).isconcretetype($T)
                ...
            end

            return $closure()::$T
        end
    end

    return combinedef(func)
end

It seems like promote_op gets the same results if you put all the function body in a closure. And, as before, the check looks to compile away if everything is type-stable.