Going from 1.0a1 to 1.0a2 should not require --pre flag.
I have mixed feelings about this. Thoughts @pypa/pip-developers ?
I can see the logic, but I'm nervous about it becoming hard to document clearly when you do and don't need --pre. Presumably, for example, 1.0a1 -> 1.1a2 should still need --pre. What about dev releases (or dev releases of pre releases)? I don't even know what --pre does with dev at the moment.
So I'm going to close this. I can see the logic behind it but ultimately I think it's going to end up with us becoming too clever and the rules for when and when a prerelease version is installed becoming hard to explain.