As far as I am aware, the standard doesn't mandate bounds checking for std::vector::operator[] and probably never will for backwards compatibility reasons. Most standard library implementations have opt-out std::vector[] bounds checking in unoptimized builds, but not in optimized builds.

I tried a toy example with GCC [1], Clang [2], and MSVC [3], and none of them emit bounds checks with basic optimization flags.

[1] https://godbolt.org/z/W5e3n5oWM

[2] https://godbolt.org/z/Pe8nPPvEd

[3] https://godbolt.org/z/YTdv3nabn