[SR-10814] Incorrect compiler error with try + autoclosure inside another closure

[swift-ci]

Previous ID	SR-10814
Radar	None
Original Reporter	nonsensery (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Environment

• macOS 10.14.5

• Xcode 10.2.1

• Swift 5.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	None
Priority	Medium

md5: cece38381f5ed1ae82c7f2f657ef1657

duplicates:

• [SR-487] Compiler fails to consider @autoclosure when satisfying try requirements #43104

Issue Description:

Steps to reproduce

1. Create a new Swift playground.

2. Paste the code below into it.

3. Attempt to run it.

func takesThrowingClosureButDoesNotRethrow(_ block: @autoclosure () throws -> Void) {
    do {
        try block()
    } catch {
        print(error)
    }
}

func throwingOperation() throws {
    // ...
}

// At the top level, of the file, this compiles fine:
takesThrowingClosureButDoesNotRethrow(try throwingOperation())

func compilesOK() {
    // Inside a function, this compiles fine:
    takesThrowingClosureButDoesNotRethrow(try throwingOperation())
}

let failsToCompile: () -> Void = {
    // Inside a closure, the compiler thinks that this line throws an error:
    takesThrowingClosureButDoesNotRethrow(try throwingOperation())
}

Expected Result

The code should compile and run.

Actual Result

The code does not compile. The compiler reports an error on the failsToCompile declaration:

Invalid conversion from throwing function of type '() throws -> ()' to non-throwing function type '() -> Void'

Discussion

The sample code contains a function – takesThrowingClosureButDoesNotRethrow – that has as an @autoclosure throwing closure argument, but does not "rethrow" errors.

Calling this function with a throwing expression from the top level of the playground, or from inside a function works as expected. But, doing the same from inside a closure results in a compiler error.

One other note: Removing the @autoclosure annotation and explicitly passing { try throwingOperation() } works in all cases.

[belkadan]

I think this one's tricky because the throws checker runs before learning that the argument expression there is an @autoclosure. I'll find the dup later.