[Sema] Look through FunctionConversionExpr when finding @discardableResult callee

[theblixguy]

Resolves #60276

@discardableResult @MainActor
func a() async -> Int { 5 }

func b() async {
  await a()
}

On 5.7. you get a warning:

<source>:3:11: warning: result of call to function returning 'Int' is unused
    await a()
          ^~~

When using @MainActor the call is wrapped in an implicit FunctionConversionExpr and we currently do not look through that to find the callee function, so the code falls through and triggers this warning.

This is a regression in 5.7, as this warning does not appear in 5.6.

[xedin]

@swift-ci please test

[LucianoPAlmeida]

MacOS test is failing on OSes that not support concurrency, I think in this case is just fine to run this with -disable-availability-checking at RUN line...

[theblixguy]

@swift-ci please test

[theblixguy]

@swift-ci please test

[theblixguy]

BTW should I create a 5.7 PR too? Since it’s a regression and it’s a very low risk change.

[xedin]

@theblixguy Please do open 5.7 PR for this.

[flowtoolz]

It has been merged on August 4, but the bug is still present in the official Xcode 14 release. May I be doing anything wrong?

Edit: Ooops, I see this has already been discussed in #60386.