[SR-13135] Type inference regression in Swift 5.3/Xcode 12 Beta

[swift-ci]

Previous ID	SR-13135
Radar	rdar://problem/65088975
Original Reporter	DeeSee (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Xcode 12 Beta 1/macOS Catalina 10.15.5 (19F101)

Additional Detail from JIRA

Votes	4
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	DeeSee (JIRA)
Priority	Medium

md5: de79c846cf09afec4f64b77b416ac261

Issue Description:

Consider this code sample:

private struct Foo {
  var bar: [Int] = []
}

let baz: Int? = nil

func foo<T: Equatable>(
  _ a: @autoclosure () throws -> T,
  _ b: @autoclosure () throws -> T
) {}

foo(Foo().bar, [baz]) 

It runs perfectly fine in Xcode 11.5 playground, while Xcode 12 Beta 1 outputs the following error:

error: test.playground:11:17: error: cannot convert value of type 'Int?' to expected element type 'Array<Int>.ArrayLiteralElement' (aka 'Int')
foo(Foo().bar, [baz])

Please note that @autoclosure attribute for foo's parameters is a required precondition for this issue.

[typesanitizer]

Seems like covariance in the return type is not working properly for autoclosures? Here's a slightly reduced example:

let x : [Int] = []
let y: Int? = nil
func f<T>(_: @autoclosure () -> T, _: @autoclosure () -> T) {}
f(x, [y])

Thanks for reporting with a small example! 🙂

[typesanitizer]

@swift-ci create

[xedin]

Fixed by #32724 Please use next available nightly build of master to verify and close.