OpenJDK 11.0.27 on macOS 15: expression is not an integral constant expression

[Fred Wright]

On Sun, 20 Apr 2025, Dave Allured - NOAA Affiliate via macports-dev wrote:
> On Sun, Apr 20, 2025 at 11:29 AM Joshua Root <jmr at macports.org> wrote:
>
>> On 21/4/2025 01:27, Nils Breunese wrote:
>>> I created a draft pull request to bump the openjdk11 port to version
>> 11.0.27 (https://github.com/macports/macports-ports/pull/28212), but it
>> looks like this line:
>>>
>>>       AO_UNUSED_MBZ = (-1)<<13, // options bits reserved for future use.
>>>
>>> in src/jdk.pack/share/native/common-unpack/constants.h results in this
>> error on macOS 15:
>>>
>>>       expression is not an integral constant expression
>>>
>>> On macOS 13 and 14 the build succeeds without this error.
>>>
>>> I am not too familiar with C++, but does this seem related to a change
>> in macOS 15? Any idea what should be done to fix the build on macOS 15?
>>
>> It would be a compiler change rather than a change in the OS. I believe
>> using bitwise shift operators on a negative value has undefined
>> behaviour. The fix would be to express the constant in a well-defined
>> way, for example directly as a hex value, or by shifting a positive
>> value and then negating afterwards. I don't know how this constant is
>> used, so I can't say what would be most appropriate.
>
>
> cppreference.com says that is undefined behavior.  That constant is defined
> as an enum, which I think means only that it needs to be a correctly
> defined integer.  Try something like this:
>
>     AO_UNUSED_MBZ = (~((1<<13) - 1))

No need to elaborately construct a two-complement negation when negation 
was never the point in the first place.  Just use:

        AO_UNUSED_MBZ = ~0<<13, // options bits reserved for future use.

Or, for a more minimal textual change:

        AO_UNUSED_MBZ = (~0)<<13, // options bits reserved for future use.

Though the parens were never necessary, since the unary operator takes 
precedence over the shift.

Fred Wright