RFC replaces directive to completely disable and replace another yml block

[tractorcow]

Given this config

---
Name: somecoreconfig
---
SomeClass:
  extensions:
    - SomeExtension
---
Name: myconfig
replaces: '#somecoreconfig'
---
SomeClass:
  extensions:
    - AnotherExtension

The "merged" config would only contain

SomeClass:
  extensions:
    - AnotherExtension

This would work similarly to before and after, but would only work with a single named block #somename.

Other blocks that are before / after the named block would not break, but the replacement would be substituted in place of the original in the original position.

You cannot use replaces with before / after

[micobarac]

This use case covers disabling extension defined by the original configuration.
To replace class name by another and attach it to the extension, the configuration should go in two steps:

• disable extension for class set by original extension configuration *.yml file
• enable the same extension for another class, other than the original one

It would be ideal to do this in the same custom /mysite/_config/*.yml file.

[tractorcow]

Yes, so in your case the usage would be:

---
Name: mycomments
replaces: '#commentssitetree'
---
SilverStripe\Blog\BlogPage:
  extensions:
    - SilverStripe\Comments\Extensions\CommentsExtension
  comments:
    enabled_cms: true

(assuming we implement this feature)