[SR-354] Non-optional variable tuple of optional values should not have default value

[swift-ci]

Previous ID	SR-354
Radar	None
Original Reporter	ianterrell (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 5b5256f99ab266dee19af877b39508b3

relates to:

• [SR-10931] Optional initialization inconsistent between Optional<T> vs T? #53322

Issue Description:

Given the following class:

class Foo {
    var bar: (String?, String?)
}

I expect it not to compile, considering bar is a non-optional type without a default value. Instead, it compiles and the synthesized initializer sets bar to (nil, nil):

let f = Foo()
f.bar
// => (nil, nil)

If the tuple is instead a constant, it does not compile:

class Foo {
    let bar: (String?, String?)
}

This results in:

tuple.swift:1:7: error: class 'Foo' has no initializers
class Foo {
      ^

Further, the messaging is not consistent, as non-tuple types communicate more information in the compiler error. With a string:

class Foo {
    let baz: String
}

the following additional error is communicated:

tuple.swift:2:9: note: stored property 'baz' without initial value prevents synthesized initializers
    let baz: String
        ^
                    = ""

While that is the stored property usage, similar behavior results in normal variable usage.

var bar: (String?, String?)
print(bar)
// => (nil, nil)

But with a constant:

let bar: (String?, String?)
print(bar)

It does not compile:

tuple.swift:11:7: error: constant 'bar.0' used before being initialized
print(bar)
      ^

If this behavior is expected, I do not see it documented in the language guide.

[belkadan]

@DougGregor, what do you think?

[Dante-Broggi]

This still occurs in the Xcode 9.4.1 toolchain.

[masters3d]

Does this need to go over evolution? This is just a weird side effect.

[belkadan]

It'd be a source-incompatible change to remove it, so it should at least start there, but the core team might decide it doesn't need a full review.