Transformation removes of inner calc

[leimonio]

ccsnano offers some configurable optimizations, like calc. Using webpack with optimize-css-assets-webpack-plugin for production builds results in transforming
calc(100vh - 5rem - calc(10rem + 100px)) to calc(100vh - 5rem - 10rem + 100px).

This is not happening though when disabling calc optimizations on default preset

preset: [`default`, { calc: false }]

To be more specific I'm trying to solve this issue gatsbyjs/gatsby#9858

(Opening issue as mentioned in cssnano/cssnano#657 (comment))

[andyjansson]

Thanks for the report. Will try to fix so it outputs the correct transformation.

[alexander-akait]

/cc @andyjansson let's fix this issue and release new version of cssnano, thanks for helping