RFR: 8377895: Create sizeof_auto to reduce narrowing conversions [v2]

[Leo Korinth]

On Mon, 23 Feb 2026 18:42:01 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> I think your solution is the very best solution. I will use it.
>
> This is probably subjective. I find this nested `conditional_t` rather hard to parse/follow. I wonder what you think of `if constexpr`.
> 
> 
> template<size_t N>
> constexpr auto sizeof_auto_impl() {
>     if constexpr (N <= 0xFF)       return uint8_t(N);
>     if constexpr (N <= 0xFFFF)     return uint16_t(N);
>     if constexpr (N <= 0xFFFFFFFF) return uint32_t(N);
>     return uint64_t(N);
> }

I much prefer that version (I tried something similar), unfortunately it does not work. I am a bit unsure exactly why, but I think your version will deduce different return types for auto, where my version will deduce the same type alias (to different types). That is just a guess though.

If you can fix it, I would prefer your version (with `std::numeric_limits<uint8_t>::max()` instead of `0xFF`)

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PR Review Comment: https://git.openjdk.org/jdk/pull/29716#discussion_r2846987196