[Diagnostics] Improve diagnostics on invalid open extensions

[mininny]

Using 'open' as default access for an extension results in the following error:

open extension AClass { // Extensions cannot use 'open' as their default access; use 'public'
// Replace 'open' with 'public'
    @objc func functionA()

Applying the autocorrect and making the function as open instead, the code is changed to this and warning is emitted:

public extension AClass {
    @objc open func functionA() // 'open' modifier conflicts with extension's default access of 'public'

where the warning holds no purpose because the 'open' modifier still works. (It seems like this is the case for other types of this conflict, i.e. public function in a private extension, and the function is still publicly visible. Similar thing can be observed in classes, i.e. a private class with a public variable, and no warning is shown there)

To remove the warning, you have to now remove the 'public' modifier like this:

extension PublicClass {
    @objc open func functionC()

I think we can improve these diagnostics to something like this to make this more convenient:

open extension AClass { // Extensions cannot use 'open' as their default access; remove the default access and mark methods as 'open' instead // remove 'open'
    @objc func functionA() // add 'open' ← maybe?

Resolves SR-15071

[varungandhi-apple]

Suggested change

	return;
	continue;

Also, to catch this, I recommend shuffling the order of members in the test case, so that we make sure we emit fix-its for later members even if we skip it for members in-between.

[mininny]

Seems logical 👍 I fixed the test cases as well :)

[varungandhi-apple]

Due to use of this import Foundation, you should use -sdk %clang-importer-sdk in the third RUN line, since we don't need the "real" Foundation, the fake one in the test suite should be good enough.

[mininny]

I didn't know that existed, thanks :) Added and pushed!

[varungandhi-apple]

@swift-ci smoke test

[mininny]

It seems like the use of import Foundation isn't passing on non-macOS platforms. Is there something I should look into?

[varungandhi-apple]

Hmm, the test needs REQUIRES: objc_interop to be able to use @objc. One thing you could do is split this test out into a new file (say open_objc.swift) which has REQUIRES: objc_interop and import Foundation. Since ObjC interop is not turned on for Linux, that test will not be run.

[xwu]

"Mark" works but I don't know that we use this term very often in Swift diagnostics. How about "declare" as above and below?

[mininny]

Seems good! Fixed 👍

[xwu]

@swift-ci smoke test

[mininny]

Can someone run the tests again please? :)

[LucianoPAlmeida]

@swift-ci smoke test