[sghill]

It does sound like they were debating this for quite a while. Eric Lippert used to work on the C# compiler and published a blog post[1] immediately after the announcement:

    Since I always knew that open sourcing was a possibility I tried to write my portions of it as cleanly and clearly as possible; hopefully I succeeded.

[1]: http://ericlippert.com/2014/04/03/c-and-vb-are-open-sourced/