Minimal reproducer

func f(_ xs: @autoclosure Any...) { return }
f(0)

Top of the stack trace when the crash happens:

4  swift                    0x000000010f48e96f swift::TypeChecker::conformsToProtocol(swift::Type, swift::ProtocolDecl*
, swift::DeclContext*, swift::OptionSet<swift::ConformanceCheckFlags, unsigned int>, swift::SourceLoc) + 95            
5  swift                    0x000000010f27dd27 swift::constraints::ConstraintSystem::getPotentialBindings(swift::TypeVa
riableType*) + 11703                           

@swift-ci create

As a work-around, you could use an array [@autoclosure Any] to get the same effect, except that you have some extra square brackets. That seems to not crash the compiler.

@swift-ci create

Comment by David Hoerl (JIRA)

theindigamer (JIRA User) Thanks for the workaround. I was playing with this as a future extension to something I use regularly (the code minus the autoclosure). I didn't think to reduce the error to the absolute minimum as you did - will work harder on future posts.

I am surprised an [@autoclosure <type_name>] works - I thought we needed a function type as an argument to @ autoclosure? Variadic autoclosures were already banned long time ago, so this looks like a regression. I'll take a look!

Here's a fix for the original problem: #28677

I've also found a different crash:

func foo(_ closure: @autoclosure () -> String...) // gets diagnosed
foo("A") // but crashes here (although it shouldn't)

I'll file a separate bug for this.

The [@autoclosure T] case should also be an error as T is not a function type, but I can create a separate bug for this as well.

@theblixguy I just filed a bug for [@autoclosure T], looks like we just allow it in any non-parameter position if it's used with a non-function type

@hamishknight Thank you! I'll investigate that further, that seems to me like a more general problem with validating @autoclosure.

Fixed on master - please verify using the next available development snapshot!

Resolved by #28819 dhoerl (JIRA User) Please use next available nightly snapshot of master to verify.