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According to the [0.5.0 release notes](https://github.com/trufflesuite/truffle/releases/tag/v5.0.0-beta.0) there are different ways to select a compiler version.
Using a version number
only seems to work with releases, and `truffle compile --list --all` also only lists them.
It would be nice to support nightly releases (in some cases), though it may be a bad idea supporting that easily via the above feature.
However, truffle also supports specifying a path, such as `version: "/home/test/mycompiler.js"`, which I think could be extended to be a URL:
In this case it could be used for nightlies and specific compiler versions (such as one with Z3 included), yet it requires a bit more work from the user compared to just dropping in a number and as a result should discourage randomly selecting a prerelease.