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[SR-10900] propertyDelegate type supersedes explicit type annotation #53290

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keith opened this issue Jun 9, 2019 · 2 comments
Closed

[SR-10900] propertyDelegate type supersedes explicit type annotation #53290

keith opened this issue Jun 9, 2019 · 2 comments
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@theblixguy
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself property wrappers Feature: property wrappers

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@keith
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keith commented Jun 9, 2019

Previous ID SR-10900
Radar None
Original Reporter @keith
Type Bug
Status Resolved
Resolution Done
Environment

Xcode 11.0 beta 1 (11M336w)

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, PropertyWrappers
Assignee @theblixguy
Priority Medium

md5: 3fcf0f62c44720d56c042465fc0bc633

is duplicated by:

  • SR-11443 Property Wrapper overwrites type declaration

Issue Description:

If you have this code:

@propertyDelegate
struct Foo<Value> {
    var value: Value
    init(thing: Value) {
        self.value = thing
    }
}

let initial: Bool? = nil
struct Bar {
    @Foo(thing: initial) var thing: Bool
}

print(type(of: Bar().thing))

it outputs:

% DEVELOPER_DIR=/Applications/Xcode-11.0.0b1.app xcrun swift foo.swift
Optional<Bool>

In this case I'm surprised that this compiles, because I would expect the explicit `Bool` type annotation to have a mismatch with the property delegate's type. This further confused me because the compiler does error if they're entirely different types:

@propertyDelegate
struct Foo<Value> {
    var value: Value
    init(thing: Value) {
        self.value = thing
    }
}

struct Bar {
    @Foo(thing: "some string") var thing: Bool
}

this outputs:

% DEVELOPER_DIR=/Applications/Xcode-11.0.0b1.app xcrun swift foo.swift
foo.swift:10:6: error: cannot convert value of type 'Foo<String>' to specified type 'Bool'
    @Foo(thing: "some string") var thing: Bool
     ^~~~~~~~~~~~~~~~~~~~~~~~~

This might be by design because I did see some example from WWDC that was relying on the inference of a type like `Bool`, yet the property ended up being a `Optional<Bool>` but this surprised me overall since I didn't expect the property with an explicit type annotation to end up possibly being a different type
@belkadan
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cc @DougGregor

@theblixguy
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Fixed by my PR: #27226

It now produces a diagnostic:

value of optional type 'Bool?' must be unwrapped to a value of type 'Bool'

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

Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself property wrappers Feature: property wrappers
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@keith @belkadan @theblixguy