[ralph]

Seemingly often enough that it deterred them with Go. No, you may not be able to revert to before the bug was introduced as it may be the bug was there ever since that feature was added. As soon as it is self-hosted, the old compiler becomes quickly irrelevant, i.e. the code rapidly diverts from what it can compile.

[pjmlp]

That is why bootstraping done properly is always done in stages.

You have a compiler that can only compile a specific subset and use that subset to write the real compiler. There are endless book examples how to do it.

Given who Go designers are, I think they don't have any issue keeping the C code around.

[ralph]

That's not how the books say to do it, and you're right, given who created Go you'd think they'd know this stuff. :-)

Many generations of the compiler are created. Let's say the compiler-in-C is worked on until it compiles subset Gosub1 which is just enough to write compiler-in-Gosub1 that duplicates compiler-in-C's behaviour. From now, compiler-in-C atrophies. G-2 features are implemented in G-1's compiler, though nothing uses them yet. The compiler's source then uses these, making it G-2 source, only compilable by a G-2-grokking compiler.

Weeks later we have a G-40 where a bug is discovered, introduced in G-20. It wasn't in the compiler-in-C so that's not useful. Choices include fixing it at `head', which can sometimes be awkward as described earlier, or fixing the initial G-20 implementation and then rolling forward all changes from there assuming the fix doesn't break code that was depending on the errant behaviour.