RFR: 8314488: Compiling the JDK with C++17

[Kim Barrett]

On Sat, 16 Aug 2025 11:34:27 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> doc/cpp17-features.md line 412:
>> 
>>> 410: [p0144r0](http://wg21.link/p0144r0)
>>> 411: [p0217r3](http://wg21.link/p0217r3)
>>> 412: 
>> 
>> It might well be worth allowing limited used of Structured Bindings. The ability to return a status from an incomplete function is something we've lacked:
>> 
>> 
>> std::tuple<double, bool> incomplete_function(double x) {
>>   if (x >= 0)
>>     return std::tuple(sqrt(x), true);
>>   return std::tuple(x, false);
>> }
>> 
>>   const auto[root, failure] = incomplete_function(d);
>>   if (failure) {...
>
> I've had variations of this conversation a number of times with various
> HotSpot folks. The upshot has always been that the preferred approach is that
> a function returning multiple values should return a named class/class
> intended for that purpose, with named and typed members/accessors. (Or
> probably even more often, a value and out parameters. Though I personally am
> not a big fan of those, esp. out reference parameters.) Names and types are
> more meaningful than offsets and implicit types. There are folks who strongly
> dislike that some of the standard library functions return `std::pair` because
> `first` and `second` members don't carry any useful information.
> 
> For the specific case of "value | error-value" I'd personally prefer we
> pursued something in the direction of boost.leaf, boost.outcome, or
> std::expected. (I think all of these can be significantly trimmed down if one
> only wants to support a fixed restricted type of error-value, like an error
> code. I think that's even the recommended style for boost.outcome?)

You are, of course, free to argue the contrary, and that we should allow the
use of structured bindings (or any other categorization in this list or
elsewhere in the style guide). If you find there is sufficient support, we can
change things. (I personally don't have a strong opinion either way about
structured bindings. I put it in the forbidden column because I know there are
some folks who would be strongly opposed. I've not had anyone express to me a
strongly favorable opinion, but that doesn't mean they aren't out there.)

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PR Review Comment: https://git.openjdk.org/jdk/pull/25992#discussion_r2280395866