[masklinn]

> So you need 1.4 to build 1.5....can you then rebuild 1.5 from 1.5?

If the language is forward-compatible, yes. It could be different if new optimisations have been added, or the code generator has changed.

> Would it make a difference if you then rebuilt 1.5 using 1.5 built using 1.5 rather than 1.5 built using 1.4?

It should not, the 1.5 compiler should be stable and yield the same result whether it was built using a 1.4 compiler or a 1.5 compiler.

That's actually a common technique to look for regressions and validate the bootstrapped compiler. For instance to compile rustc you download a "stage 0" compiler at Vx which compiles a "stage 1" compiler using Vy > Vx source. That stage 1 compiler is then used to compile the stage 2 compiler from the same source. Stage 1 and stage 2 may not be identical since stage 0 and stage 1 may have different optimisations &al, so the stage 2 compiler is used to compile rust a third time, and that compilation is checked against the original stage 2 (and should be a fixpoint).