[swift-evolution] [proposal] Allow trailing closures in 'guard' conditions

[Brent Royal-Gordon]

> The solution is simple: allow trailing closures in guard bodies.  This would allow this silly example to compile correctly:
> 
> func f(arr : [Int]?) {
>   guard let x = arr?.map {$0+1} else {
>     preconditionFailure()
>   }
> 
>   // ...
> }

This sounds perfectly reasonable.

> Alternatives considered
> 
> There are three primary alternatives: do nothing, expand the scope of ‘if' and ‘while’ conditions as well, and significantly change the syntax of guard.

A fourth alternative would be to change the syntaxes of `if` and `while` (and probably `for` and `switch`) to also have a keyword in this position.

	if expr then { code }
	while expr do { code }
	for elem in expr do { code }
	switch expr among { code }

I'm not going to say I advocate for this option, but it *would* clearly mark the end of the condition so that trailing closures could be brought to all of these statements, so it seemed worth mentioning.

-- 
Brent Royal-Gordon
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