How so? There are compiler-agnostic C++ specs, and compiler devs try to be compatible with it.

What the GP is suggesting is that the rust compiler should be written and then a spec should be codified after the fact (I guess just for fun?).

> compiler devs try to be compatible with it.

You have to squint fairly hard to get here for any of the major C++ compilers.

I guess maybe someone like Sean Baxter will know the extent to which, in theory, you can discern the guts of C++ by reading the ISO document (or, more practically, the freely available PDF drafts, essentially nobody reads the actual document, no not even Microsoft bothers to spend $$$ to buy an essentially identical PDF)

My guess would be that it's at least helpful, but nowhere close to enough.

And that's ignoring the fact that the popular implementations do not implement any particular ISO standard, in each case their target is just C++ in some more general sense, they might offer "version" switches, but they explicitly do not promise to implement the actual versions of the ISO C++ programming language standard denoted by those versions.