[BUG] diff('2.0.0-alpha.15', '2.0.0') returns prerelease when it should return major

[lazd]

What / Why

A bump from an alpha of a major release to the actual major release is considered a prerelease by diff when it's actually a major release.

When

• Always

Where

• npm/semver

How

Current Behavior

• diff('2.0.0-alpha.15', '2.0.0') returns prerelease

Steps to Reproduce

const semver = require('semver');
semver.diff('2.0.0-alpha.15', '2.0.0'); // == prerelease

Expected Behavior

• diff('2.0.0-alpha.15', '2.0.0') returns major
• That is, the output of diff('2.0.0-alpha.15', '2.0.0') should be consistent with inc('2.0.0-alpha.15', 'major').
• When we increment 2.0.0-alpha.15 by major, we get 2.0.0, and when we diff 2.0.0-alpha.15 against 2.0.0, we should also get major.

Who

• n/a

References

• diff(v1, v2): difference in versions, such as major, minor, patch #83
• diff(v1, v2) #111

[lukekarrys]

Closing as a duplicate of #333 which we'll use to track this bug.