I think you are confusing specification with implementation. The C++ standard does not outline how a compiler implements a feature, just how it should behave to match the standard. There are several compilers that 100% adhere to the full standard, including the guarantees specified by the standard in the form of memory and time complexity (optimization).

Oh, I know that specification != implementation, but IIRC both clang and GCC have certain behaviour which do not conform to the standard. But I can't find these right now, so I'll drop it here :)