Please don't be pedantic. Compilers implement the standard, otherwise it's just a text document.

Not trying to be pedantic. When I hear "leave it to the compiler", I normally think, "let the compiler optimize it, rather than optimizing it yourself". The compiler is doing the initialization either way, but in one case you're relying on a correct understanding of minutiae of the language spec (both for you and all future readers and writers of the code), in another case you're explicitly instructing the compiler to initialize it to zero.

Compilers implement the parts of the standard they agree with, in the way they think is best. They also implement it in the way they understand the standardese.

Read a complex enough project that's meant to be used across compiler venrdos and versions, and you'll find plenty of instances where they're working around the compiler not implementing the standard.

Also, if you attended the standards committee, you would hear plenty of complaints from compiler vendors that certain things are implementable. Sometimes the committee listens and makes changes, other times they put their fingers in their ears and ignore reality.

There are also plenty of places where the standard lets the compiler make it's own decision (implementation defined behavior). You need to know what your compiler vendor(s) chose to do.

tl;dr: With a standard as complex as C++'s, the compilers very much do not just "implement the standard". Sometimes you can get away with pretending that, but others very much not.