RFR: 5038439: Warning message for literal shift amounts outside the canonical domain [v2]

[Hannes Greule]

On Mon, 3 Nov 2025 18:32:57 GMT, Archie Cobbs <acobbs at openjdk.org> wrote:

>> When bit shifting an `int` or `long` value by an amount `X`, all but the last 5 or 6 (respectively) bits of `X` are ignored.
>> 
>> This can create a trap for the unwary, as in this example:
>> 
>> public long readLongBigEndian(byte[] buf, int offset) {
>>     return ((buf[offset + 0] & 0xff) << 56)   // BUG HERE
>>          | ((buf[offset + 1] & 0xff) << 48)   // BUG HERE
>>          | ((buf[offset + 2] & 0xff) << 40)   // BUG HERE
>>          | ((buf[offset + 3] & 0xff) << 32)   // BUG HERE
>>          | ((buf[offset + 4] & 0xff) << 24)
>>          | ((buf[offset + 5] & 0xff) << 16)
>>          | ((buf[offset + 6] & 0xff) << 8)
>>          | ((buf[offset + 7] & 0xff);
>> }
>> 
>> This PR adds a new warning when the compiler detects an out-of-range bit shift, i.e., an `int` bit shift not in the range `[0...31]` or a `long` bit shift not in the range `[0...63]`.
>
> Archie Cobbs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:
> 
>  - Merge branch 'master' into JDK-5038439 to fix conflict.
>  - Address review comment (combine switch cases).
>  - Merge branch 'master' into JDK-5038439 to fix conflict.
>  - Add "long" as a supported message parameter type.
>  - Use "bit(s)" instead of "bits" where value could be 1.
>  - Merge branch 'master' into JDK-5038439
>  - Sprinkle more variety into the regression test.
>  - Minor diff cleanup.
>  - Update "lossy-conversions" description in compiler module Javadoc.
>  - Warn for bit shifts using an out-of-range shift amount.

test/langtools/tools/javac/lint/ShiftOutOfRange.java line 18:

> 16:         a = a >>> (short)-1;
> 17:         a <<= -1;
> 18:         a >>= -1L;          // also generates "implicit cast from long to int in compound assignment is possibly lossy"

I'm a bit surprised by the additional warning here. The shift still operates on int, not long; i.e., unary promotion is performed for each argument rather than binary promotion https://docs.oracle.com/javase/specs/jls/se25/html/jls-15.html#jls-15.19

Should this be fixed (separately, obviously)?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27102#discussion_r2487472103