[SR-9626] Unexpected behavior of static constructor's type inference #52072

swift-ci · 2019-01-09T07:03:47Z


Previous ID	SR-9626
Radar	rdar://problem/47324309
Original Reporter	domas (JIRA User)
Type	Bug
Status	Closed
Resolution	Done

Environment

macOS Mojave Version 10.14.2 (18C54)
Xcode Version 10.1 (10B61)
Apple Swift version 4.2.1 (swiftlang-1000.11.42 clang-1000.11.45.1)

Additional Detail from JIRA


Votes	0
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	domas (JIRA)
Priority	Medium

md5: b4bfa621d7cc84553991cf9637b2262b

Issue Description:

1. Given a base class, it's subclass, a generic struct and a generic func we can have this arrangement:

class BaseClass {}
class SubClass: BaseClass {}
struct Box<T> { init(_: T.Type) {} }

func test<T>(box: Box<T>) -> T.Type {
    return T.self
}

test(box: .init(SubClass.self)) // Results with expected `SubClass.Type`

This works as expected. Though there are 2 easy ways to break the result of `func test`.

2.1. Adding a type constraint on `func test`:

class BaseClass {}
class SubClass: BaseClass {}
struct Box<T> { init(_: T.Type) {} }

func test<T: BaseClass>(box: Box<T>) -> T.Type {
    return T.self
}

test(box: .init(SubClass.self)) // Unexpectedly results with `BaseClass.Type`

2.2. Adding a type constraint on `Box`:

class BaseClass {}
class SubClass: BaseClass {}
struct Box<T: BaseClass> { init(_: T.Type) {} }

func test<T>(box: Box<T>) -> T.Type {
    return T.self
}

test(box: .init(SubClass.self)) // Unexpectedly results with `BaseClass.Type`

What's even more unexpected is that adding another func named `test` to 2.1. and 2.2. changes the result back to `SubClass.Type`:

3.1. Adding anoter `func test` to 2.1:

class BaseClass {}
class SubClass: BaseClass {}
struct Box<T> { init(_: T.Type) {} }

func test<T: BaseClass>(box: Box<T>) -> T.Type {
    return T.self
}

func test(someParameter: Any) {}

test(box: .init(SubClass.self)) // Now results with `SubClass.Type`

3.2. Adding anoter `func test` to 2.2:

class BaseClass {}
class SubClass: BaseClass {}
struct Box<T: BaseClass> { init(_: T.Type) {} }

func test<T>(box: Box<T>) -> T.Type {
    return T.self
}

func test(someParameter: Any) {}

test(box: .init(SubClass.self)) // Now results with `SubClass.Type`

belkadan · 2019-01-09T17:12:16Z

I can see why there's an ambiguity, because SubClass.self is a valid BaseClass, but it certainly does seem like the behavior should be consistent here. cc @xedin

xedin · 2019-01-17T19:21:21Z

Thank you for the report, domas (JIRA User)! I've merged PR #21931 which is going to resolve this issue, please use the next available master nightly build to verify and close this issue.

swift-ci transferred this issue from apple/swift-issues Apr 25, 2022

This issue was closed.

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[SR-9626] Unexpected behavior of static constructor's type inference #52072

[SR-9626] Unexpected behavior of static constructor's type inference #52072

swift-ci commented Jan 9, 2019

belkadan commented Jan 9, 2019

xedin commented Jan 17, 2019

[SR-9626] Unexpected behavior of static constructor's type inference #52072

[SR-9626] Unexpected behavior of static constructor's type inference #52072

Comments

swift-ci commented Jan 9, 2019

belkadan commented Jan 9, 2019

xedin commented Jan 17, 2019