<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 7, 2017, at 10:12 AM, Letanyan Arumugam via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span style="font-family: SourceCodePro-Regular; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">My original argument about failing for no discernible reason still stands. precondition is a thought out check for a failing state. member lookup failures are going to be because things don’t exist for which you presumably expected to exist.</span></div></blockquote></div><br class=""><div class="">Is it necessarily true that things will have to fail "for no discernible reason"? It's up to the implementers of the protocol for how they want to handle the failure case (trap/return option/throw error/etc.). But I imagine it could be possible to add a generic debug error message/warning message about lookup failing for invocations of this kind, which would give a pretty clear hint about what went wrong and why. Would that be enough to address your concerns about silent failures?</div></body></html>