ci: Bump GCC_SNAPSHOT_MAJOR to 15

[maflcko]

Follow-up to #1313

Clang should silently follow the main devel branch, but GCC needs to be bumped manually.

[real-or-random]

GCC 15 has a new warning, see https://gcc.gnu.org/pipermail/gcc-patches/2024-June/656014.html .

[maflcko]

Yeah, I guess one solution would be to be verbose and write the array:

static const unsigned char PREFIX[19] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', ' ', 't', 'e', 's', 't', ' ', 'i', 'n', 'i', 't'};

an alternative would be to disable the warning, which seems cleaner and any issues that the warning theoretically could uncover should be detectable by a the existing CI sanitizer builds?

[real-or-random]

static const unsigned char PREFIX[19] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', ' ', 't', 'e', 's', 't', ' ', 'i', 'n', 'i', 't'};

This is ultimately a question of taste, but I suggest this because I think it's the most readable:

static const unsigned char PREFIX[] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', ' ', 't', 'e', 's', 't', ' ', 'i', 'n', 'i', 't'};

This makes it unambiguously clear that we consider it an array and not a string (i.e., it's not null-terminated), the reader doesn't need to count the chars, and we can use sizeof(PREFIX).

We define such constant arrays in multiple code locations, see
https://github.com/bitcoin-core/secp256k1/actions/runs/10356727938/job/28667982412?pr=1583
Whenever I review such code, I need to count the chars manually to check whether the terminating null byte is included or not.

[maflcko]

Whenever I review such code, I need to count the chars manually to check whether the terminating null byte is included or not.

Good point about the review overhead and ambiguity. I pushed a commit with your suggestion.

[hebasto]

ACK fa67b67, I have reviewed the code and it looks OK.

I can see gcc version 15.0.0 20240811 (experimental) (GCC) in the CI logs.

[real-or-random]

utACK fa67b67