The calculation for 256^8 using Doubles works as expected:
(lldb) po pow(Double(256), Double(8))
1.8446744073709552e+19
However, trying to convert the result into a Decimal crashes:
(lldb) po Decimal(pow(Double(256), Double(8)))
error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).
The process has been returned to the state before expression evaluation.
Decimals should handle values with up to 38 digits as mantissa with exponents from –128 through 127.
Apple feedback id: FB7706665