The documentation [1] for core.bitop.bt just says "Tests the bit."
With the function returning an int, the question is whether the return value is guaranteed to be 0 or 1, as ensured by the current implementation, or whether it can be any non-zero integer if the bit is set (as documented for the related bts function).
The std.bitmanip.BitArray formatting functions in 2.064 assume the former, and are thus currently broken with LDC. LDC will probably end up switching to simply using the D implementation anyway as LLVM recognizes the pattern just fine, but I'd like to know what to fix first.
David [1] http://dlang.org/phobos/core_bitop.html
