Extension causes "Type could not be found" error

[disarray2077]

Any extension causes the error below in SplitList<T>:

ERROR: Type could not be found (are you missing a using directive or library reference?) at line 1:1 in $Emit$corlib:System.Collections.SplitList<>
int_cosize[0] mOffsets;
^^^^^^^^^^

Example extension code:

namespace System.Collections
{
	public extension SplitList<T>
	{
	}
}

Tested with: https://github.com/beefytech/Beef/commit/aaa08e9a4e9d217b58ea9f5c5fd81f1053d60a10