Iteration on UTF/UNICODE characters

[Danny Arends via D.gnu]

Hey all,

I am building a font glyph atlas for 2D and 3D rendering using 
the following code:

dchar c = '\u0000';
while (c <= '\U0000FFFF') {
  if(isValidDchar(c)){
    # Code working on the char
  }
}

This compiles, and works as expected using DMD, and LDC2.

However when I try and build the code using GDC, it gives me the 
following error:

src/font.d:95:15: error: invalid UTF character \U0000FFFF

Any thoughts on why this is happening in GDC, since it works in 
DMD and LDC2 ?
And any idea how to fix this ?

Kind regards,
Danny