[SR-15344] Trailing closure in guard condition gives inscrutable error

[RonAvitzur]

Previous ID	SR-15344
Radar	None
Original Reporter	@RonAvitzur
Type	Bug

Environment

Version 13.0 beta 5 (13A5212g)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 668dc78ec042659d0b3a118604c2d487

Issue Description:

guard samples.contains{ $0.isInfinite } else { return }

gives

Anonymous closure argument not contained in a closure
Cannot convert value of type '((Sample) throws -> Bool) throws -> Bool' to expected condition type 'Bool'
Closure expression is unused
Consecutive statements on a line must be separated by ';'
Expected 'else' after 'guard' condition

For comparison, a trailing closure in an if condition gives:

{{ if samples.contains { $0.isInfinite } { return }}}}}{}}}

{{ ~^}}

{{ (where: )}}

While guard is using the same condition grammar that if uses, which prohibits the trailing closure here, since guard has no then clause the trailing closure is not actually ambiguous, so it either requires a slightly different diagnostic, or a relaxation of the grammar to allow the trailing closure in a guard condition.

[rintaro]

if case is actually not "ambiguous" either.
For 'if something { ... } { ... }', the first block can't be a then block because then blocks can't have a trailing block without `else`, also the second block can't be a trailing closure because multiple trailing closure requires labels. But it's is still confusable. That's why the diagnostic is a warning, and recommends using a parenthesized argument.

guard case is similar. It is not ambiguous, but confusable. So guard case should be diagnosed as a warning just like if.