How to reproduce:
1. Download attached project files and open "testPrecision" scene
2. Enter Play Mode and press "Test" button
3. In Console Window Observe that the distance is 0.995, but should be 1
4. Notice the distance 2 3 4 in Console Window when the boxes are touching only the edges

Actual result: Physics2d Boxcast distance is less than it should be. Box coordinates are (2,0), RaycastHit2D hit2d = Physics2D.BoxCast(new Vector2(0f, 0f), new Vector2(1f, 1f), 0, new Vector2(1f, 0)); The distance is 0.995, but it should be 1.

Reproduced with: 5.5.4p3, 5.6.3p4, 2017.1.1p3, 2017.2.0f1, 2017.3.0b2