What changed?

All of these were either invalid in C99, invalid even in C89, or extremely dubious. Compilers just tolerated them as quasi-extensions until now to avoid disruption.

    Clang 15 makes the following errors by default:
        -Werror=int-conversion

    Clang 16 (released March 2023) makes the following errors by default:
        -Werror=implicit-function-declaration
        -Werror=implicit-int
        -Werror=incompatible-function-pointer-types (GCC does not have a specific equivalent error (PR109835), use -Werror=incompatible-pointer-types instead when testing)

    GCC 14 (to be released appx. April/May 2024) makes the following errors by default:
        -Werror=int-conversion
        -Werror=implicit-function-declaration
        -Werror=implicit-int
        -Werror=incompatible-pointer-types
        -Werror=return-mismatch ('new' warning in GCC 14, split out from -Wreturn-type)
        -Werror=declaration-missing-parameter-type (new warning in GCC 14)