What is "dual-context" ?

[SrMordred via Digitalmars-d-learn]

source\app.d(37,10): Error: function `app.main.match!((some) => 
print(some), (none) => print("none")).match` requires a 
dual-context, which is not yet supported by LDC

I was playing with my home made "sumtypes".
The code below works fine in dmd, but in ldc it triggers that 
error.

x.match!(some => print(some), none => print("none"));


What exactly is this "dual-context"?