#71870 Respect @discardableResult when calling with try?

[giginet]

When functions are called with optional try (try?) raise warnings even if the function annotates @discardableResult.

@discardableResult
func foo() throws -> Int { 1 }

func test() {
  try? foo() // warning: result of 'try?' is unused [expression_unused_optional_try]
}

@discardableResult should be respected in such a case.

This PR changes the logic to raise the warning after checking discardableResult was applied.

warning is suppressed with this change.

Resolves #71870
@discardableResult works with try and try!, but not try? · Issue #71870 · apple/swift

[giginet]

The function call expr of try? foo() is wrapped in OptionalTryExpr and InjectIntoOptionalExpr. It need to unwrap them.

(optional_try_expr type="Int?" location=issue.swift:5:8 range=[issue.swift:5:3 - line:5:12] thrown_error="<null>"
  (inject_into_optional implicit type="Int?" location=issue.swift:5:8 range=[issue.swift:5:8 - line:5:12]
    (call_expr type="Int" location=issue.swift:5:8 range=[issue.swift:5:8 - line:5:12] isolation_crossing="none"
      (declref_expr type="() throws -> Int" location=issue.swift:5:8 range=[issue.swift:5:8 - line:5:8] decl="issue.(file).foo()@issue.swift:2:6" function_ref=single)
      (argument_list))))

[jameesbrown]

wonder if it would work the same if you move this check into the while condition? i.e:

while(nextExpr != nullptr)

[giginet]

@Jamezzzb Fixed. Thanks

[ktoso]

@swift-ci please smoke test

[AnthonyLatsis]

@giginet Please refrain from prefixing GitHub issue numbers with "SR". An SR identifier is a distinct identifier that was given to a report in the old Jira bug tracker, so only old issues that date back to Jira and were migrated have such an identifier — in addition to the GitHub issue number.

[xedin]

Thank you for the contribution! I think this change requires Swift Evolution (you can start by pitching it in Swift Evolution -> Pitches category). The problem here is semantic - try? actually produces an optional value that wraps the result of the call where nil case indicates that an error has been thrown, which is semantically distinct from the result of the function itself, that's why i.e. getSemanticsProvidingExpr() is not looking through OptionalTryExpr.