Error: llroundl cannot be interpreted at compile time, because it has no available source code

[mw via Digitalmars-d-learn]

Hi,

I'm trying to build this package:
https://code.dlang.org/packages/fixed

however, the compiler error out:

ldc2-1.21.0-linux-x86_64/bin/../import/std/math.d(5783,39): 
Error: llroundl cannot be interpreted at compile time, because it 
has no available source code

Looks like it does some CTFE, but

1) I cannot find where CTFE happens in:

https://github.com/jaypha/fixed/blob/master/src/jaypha/fixed.d


2) even it does so, but why such simple function as lroundl 
cannot be CTFE-ed?