[swift-evolution] Disallowing unreachable code

[Joe Groff]

> On Mar 29, 2017, at 4:21 PM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> On Mar 29, 2017, at 4:14 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
>> 
>>> On Mar 29, 2017, at 8:11 AM, John McCall via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> I was suggesting that it would be a useful addition to the language, not that it
>>> necessarily needed new compiler support.
>> 
>> Personally, what I'd like to see is for the existing <#whatever#> placeholder syntax to be treated as an unimplemented() call. That probably *would* require compiler support, although fortunately we already parse this syntax into an EditorPlaceholderExpr.
> 
> Actually, looking more closely, we already have this behavior in playgrounds (and REPLs); it's only an error when you compile. Like, we literally do this:
> 
>       // Found it. Flag it as error (or warning, if in playground mode) for the
>       // rest of the compiler pipeline and lex it as an identifier.
>       if (LangOpts.Playground) {
>         diagnose(TokStart, diag::lex_editor_placeholder_in_playground);
>       } else {
>         diagnose(TokStart, diag::lex_editor_placeholder);
>       }
> 
> Could we change the compile-time error into a warning? Would that require an evolution proposal?

Ah, you beat me to it. I would support reducing this to a warning.

-Joe