[DRAFT] Replacing 'synthesized initializers' with 'default initializers' on diagnostics

[Visckmart]

As highlighted by this issue, the official documentation doesn't seem to use the word 'synthesized initializer' as some of the diagnostics do. It, instead, uses 'default initializers' to refer to this concept, as can be seen at the The Swift Programming Language (5.9) > Initialization > Default Initializers section.

This pull request simply replaces one term by the other on 3 diagnostics, which are related to eachother and seem to be the only ones using the 'synthesized' term.

Resolves #43376

[Visckmart]

I was just testing the pull request on my own fork before requesting a real one soon, sorry for the review request, it was unintentional!

[Visckmart]

I think it's relevant to discuss a more thorough rewriting of this note.
The pull request currently simply replaces one word with the other, so we end up with:
stored property 'example' without initial value prevents default initializers

Which is good because we are now using the same term as the documentation. But I would like to propose changing the message to:
no default value on stored property 'example'; default initializer cannot be provided

The reasons behind some of the changes are based on the two following excerpts from
The Swift Programming Language (5.9) > Initialization > Default Initializers:

Swift provides a default initializer for any structure or class that provides default values for all of its properties [...]

[...] ShoppingListItem automatically gains a default initializer implementation [...]

1. Replacing initial value with default value:

   The documentation calls them default values, so I think the diagnostic should do the same.

2. Removal of the plural on the word "initializer":

   I think the plural on "initializers" on the original message is incorrect. As I understand, it makes no sense to have more than 1 default initializer on the same type.

Finally, two additional reasons are that to me the proposed message is more clear than the original and it will make it easier to learn about the concepts mentioned using the official documentation.

[AnthonyLatsis]

As Jordan notes here, the compiler term synthesized initializers encompasses both default and inherited initializers.

What if we rearchitect the diagnosis to avoid these terms altogether by pointing out that the class must implement an initializer because it has stored properties without default values? As a bonus, I believe this formulation would make for a far more helpful error message than the current class has no initializers. The notes could then be adjusted to simply point at these stored properties instead of spelling out causal relationships.

[Visckmart]

I totally overlooked his comment! Thank you for highlighting it.

I'm not confident in my understanding of the relation with the inherited initializers, so I'll give an example and walk through it to check if I understood correctly.

class Superclass {
    let boolean: Bool
    // The compiler will synthesize a default initializer like init(boolean: Bool)
}
class Subclass: Superclass {
    let text: String = "Example Text"
    // The compiler will synthesize an inherited initializer like init(boolean: Bool), like the one seen above
}

If the text variable didn't have an initial value, the inherited initializer wouldn't be able to be synthesized. But the changes I've made to the diagnostics would call the inherited initializer a default initializer, which is wrong.

Did I use the correct terms everywhere?

The solution then is to check if we are synthesizing an inherited initializer or a default initializer. And your suggestion is to change the current diagnostics to something along the lines of:
error: an initializer must be implemented because this class has stored properties without initial values
note: 'text' has no initial value

Did I understand the solution you proposed correctly?

[AnthonyLatsis]

If the text variable didn't have an initial value, the inherited initializer wouldn't be able to be synthesized. But the changes I've made to the diagnostics would call the inherited initializer a default initializer, which is wrong.

Yes, except a default initializer is an implicit initializer with no parameters. Memberwise initializers like init(boolean: Bool) are synthesized only for structures. Inherited initializers can have parameters and are somewhat harder to define in terms of what actually happens. For a designated initializer, the inherited initializer is a distinct, implicit override thereof. Otherwise (convenience init), it is the superclass initializer itself, and is called inherited because you can call it on the subclass. It’s easiest to simply think of them as initializers that are inherited per initializer inheritance rules.

Did I understand the solution you proposed correctly?

Sounds good!

[AnthonyLatsis]

note: 'text' has no initial value

We spell these sorts of notes as <description> declared here. It’s better not to phrase it as a complaint because we don’t know what course of action is best for the programmer, and we’d rather not mislead them.

[Visckmart]

I think I got it! Will try to implement and come back with it.

And regarding the note: makes sense, will use it.