Comment by Ky (JIRA)

I think that should be a failable initializer, returning nil for any input it cannot precisely represent, or at least that would overflow

Decimal is arbitrary-precision; the thing that "overflows" is the literal (which is interpreted as a Double). But I wouldn't expect it to truncate either. @jckarter, do you understand what's going on?

You described it accurately. Swift's literal initialization model for decimal literals is to pass them as floats. Ideally Decimal would not have provided this initializer until we had proper support for accurate decimal literals, but that's ultimately SR-3317.