Windows 10 64-bit, Geforce GTX 970, Intel Core i7-6700k, 16GB Ram
Date: 2018-08-11 18:54
Short description of error
The Glare effect in Compositing does not work properly in both Cycles and Eevee when using transparency. This issue also exists in 2.79 versions.
Exact steps for others to reproduce the error
Select the render engine of choice, choose PNG for the output with alpha included, enable the Scene's transparency in Film, then render image.