Most methods in core.atomic are versioned out with a stub method replacing them when generating documentation. If you attempt to use something like atomicOp for example, it will still compile but do nothing and always return T.init (leading to quite a bit of confusion). If you try to use cas, you get a symbol undefined, as the actual implementation is excluded when version(D_Doc) is set.

Test case:

module Test;
import std.stdio;
import core.atomic;

void main() {	
	int* a = new int();
	*a = 2;
	int* b = new int();
	*b = 1;	
	bool Result = cas(cast(shared)&a, cast(shared)a, cast(shared)b);	
	writeln(Result, ": ", *a, ", ", *b);
}

Compile with 'dmd test.d' and it runs and prints "true: 1, 1". Compile with 'dmd test.d -D' and it will fail to link with "Error 42: Symbol Undefined _D4core6atomic20__T3casTPOiTPOiTPOiZ3casFPOPiOxPiOPiZb".