Currently in pub, a constraint like "<1.0.0" will allow "1.0.0-dev". In practice, this rarely matters because it will try any non-prerelease version before considering 1.0.0-dev, but I think it's still goes against the user's intuition.

It seems totally wrong to allow an experimental unstable version of something whose stable version you don't allow. Npm's interpretation is:

>1.2.3 Greater than a specific version.

<1.2.3 Less than a specific version. If there is no prerelease tag on the version range, then no prerelease version will be allowed either, even though these are technically "less than".

>=1.2.3 Greater than or equal to. Note that prerelease versions are NOT equal to their "normal" equivalents, so 1.2.3-beta will not satisfy this range, but 2.3.0-beta will.

<=1.2.3 Less than or equal to. In this case, prerelease versions ARE allowed, so 1.2.3-beta would satisfy.

I think we should change pub to match this. I don't think it will impact many users because pre-release versions are already deprioritized by the solver.