GCC rarely, like never, supports the complete standards. Every release improves conformance, fixes errors, sometimes drops support for targets, perhaps changes how poorly defined areas work.

As a result, likely no version compiles code the same as a previous version across all possible code.

You’ll also note GCC doesn’t likely compile the variants of C++ it did before the 1998 version.

And this type of page is just the tip of the iceberg https://gcc.gnu.org/gcc-13/porting_to.html. You’ll note changes made that can render previously compilable code no longer compilable.

So no, you cannot naively think a new compiler version will simply run all your old code. On big projects upgrading is a huge endeavor to minimize problems.

We could do every supported language the same way.