[swift-evolution] [Pitch] Remove type-inference for stored property

[Seán Labastille]

Off the top of my head I’m not sure if there is a precedent for such a drastic change in behaviour between playgrounds and non-playground compilation.
However, allowing type inference in playgrounds while disallowing this otherwise could maintain the benefits for educational and experimental scenarios while still eliminating the performance and clarity penalties in production (compiled) source files.

> Hi all,
> 
> In a discussion about inferring parameter types from default value, Slava brought up some performance problems caused by type inference for stored properties in side types:
> 
> https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170313/033882.html
> 
> Towards the end, the post mentioned that some Swift team members contemplated requiring types for stored properties in type declarations. I think this idea deserves some more attention. Hence this last minute idea-floating.
> 
> In addition to solving a performance headache in implementation, there're always the general benefit of making type declartion more explicit and readable (clarity for reader should out-weigh pleasure of the author). Making the
> language slightly more consistent (we are not inferring types for default parameter values in function anyways).
> 
> The cons for doing this are obvious too: the inference makes the language feels more friendly and is, undoubtedly, a beloved feature for many. This would be a source breaking change.
> 
> Just thought I'd float the idea to gather some quick reaction. What do y'all think?
> 
> Daniel Duan
> 
> 
>