`#if swift(...)` nested within `#if canImport` tries to parse attributes

[dnadoba]

Describe the bug
This does not compile:

#if canImport(SomeModuleThatDoesNotExists)
struct Foo {}
#if swift(>=5.99)
extension Foo: @someUnkownAttribute {} // error: Expected type
#endif
#endif

But this compiles just fine

struct Foo {}
#if swift(>=5.99)
extension Foo: @someUnkownAttribute {} 
#endif

as well as this:

struct Foo {}

#if swift(>=5.99) && canImport(SomeModuleThatDoesNotExists)
extension Foo: @someUnkownAttribute {}
#endif

Steps To Reproduce
try to compile the above code

Expected behavior
All variations compile without errors

Environment (please fill out the following information)

• OS: 13.0 Beta (22A5331e)
• Xcode Version 14.0 beta 6 (14A5294g) but with Toolchain: Swift Development Snapshot 2022-08-18 (a)
• also tested in docker with swiftlang/swift:nightly-main-focal

Additional context
Original issue found in the Swift 5.4 compiler with a code snipped similar to this:

#if canImport(Network)
class Foo {}

#if swift(>=5.5) && canImport(_Concurrency)
extension Foo: @unchecked Sendable {} // error: unknown attribute 'unchecked'
#endif
#endif

We now use this workaround which compiles just fine with Swift 5.4:

#if canImport(Network)
class Foo {}
#endif

#if swift(>=5.5) && canImport(_Concurrency) && canImport(Network)
extension Foo: @unchecked Sendable {}
#endif

[hassila]

Also encountered what seems to be the same issue with Swift 5.7:

#if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
   import CDarwinOperatingSystemStats
   import Darwin
   import Dispatch

#if swift(>=5.8)
@_documentation(visibility: internal)
#endif
   final class OperatingSystemStatsProducer {
       var nsPerMachTick: Double
       var nsPerSchedulerTick: Int

       let lock = NIOLock()
...
#endif

(Swift 5.8 seems ok - fail here: https://swiftpackageindex.com/builds/E19A3504-F6EA-49D3-B0C3-31A7624991ED)

In my case it is #if swift() nested in #if os() though, so just similar, but not identical.

[AnthonyLatsis]

@dnadoba Apparently this was fixed a while ago. Your examples from the additional context compile fine, and the other three are rejected, which I believe to be expected, since extension Foo: @someUnkownAttribute {} is not a valid syntactic production.

Tested with Swift version 5.9-dev (LLVM 91080e65df791d6, Swift c1d5118).

[AnthonyLatsis]

@hassila Swift 5.7 does not support #ifs around attributes. SE-0367 was implemented in 5.8, hence your observation.

[dnadoba]

@AnthonyLatsis I'm not sure I follow.
This compiles:

struct Foo {}
#if swift(>=5.99) && canImport(SomeModuleThatDoesNotExists)
extension Foo: @someUnkownAttribute {}
#endif

but this doesn't:

#if canImport(SomeModuleThatDoesNotExists)
struct Foo {}
#if swift(>=5.99)
extension Foo: @someUnkownAttribute {} // error: Expected type
#endif
#endif

The only difference is that we are nesting the #ifs.
(tested with swift-driver version: 1.80.1 Apple Swift version 5.9 (swiftlang-5.9.0.108.11 clang-1500.0.19.2))

Even if it isn't valid syntax, shouldn't it be consistently rejected?