Comment by ndiddy
> How much code actually uses unions this way?
I see this change caused Mbed-TLS to start failing its test suite when compiled with GCC 15: https://github.com/Mbed-TLS/mbedtls/issues/9814 (kinda scary since it's a security library). Hopefully other projects with less rigorous test suites aren't using {0} in that way. The Github issue mentions that Clang tried a similar optimization a while ago and backed it out after user complaints, so maybe the same thing will happen with GCC.
GCC’s developers have a strong insistence on standards conformance (minus situations where they explicitly choose to deviate, like type punning in unions) over the status quo. We already went through a much more severe shift with strict aliasing enforcement by GCC and they never changed course. I do not expect this to be any different.