For optimization purposes, modern compilers will generally treat non-constexpr if-statements that have constexpr conditionals as if they were constexpr-if-statements. However, they are not required to do so.

A compiler that encounters a non-constexpr if-statement with a constexpr conditional may issue a warning advising you to use if constexpr instead. This will ensure that compile-time evaluation will occur (even if optimizations are disabled).