When generating a stub fix-it for a protocol conformance or implementation extension, Swift will now evaluate whether the context allows the declaration of stored properties and, if so, will suggest one. It will also use the `let` keyword instead of `var` if the property has no setter.
Changes the diagnostics emitted when an `@objc @implementation` extension is missing some of the members required by the extension:
• We now emit one error on the extension, plus a note for each missing member.
• Where possible, we also emit a note with a fix-it adding stubs.
For example:
```
9 | @objc @implementation extension ObjCClass {
| |- error: extension for main class interface does not provide all required implementations
| |- note: missing instance method 'method(fromHeader3:)'
| |- note: missing instance method 'method(fromHeader4:)'
| |- note: missing property 'propertyFromHeader7'
| |- note: missing property 'propertyFromHeader8'
| |- note: missing property 'propertyFromHeader9'
| |- note: missing instance method 'extensionMethod(fromHeader2:)'
| `- note: add stubs for missing '@implementation' requirements
```
With a fix-it on the last note to insert the following after the open brace:
```
@objc(methodFromHeader3:)
open func method(fromHeader3 param: Int32) {
<#code#>
}
@objc(methodFromHeader4:)
open func method(fromHeader4 param: Int32) {
<#code#>
}
@objc(propertyFromHeader7)
open var propertyFromHeader7: Int32 {
get {
<#code#>
}
set {
<#code#>
}
}
@objc(propertyFromHeader8)
open var propertyFromHeader8: Int32 {
get {
<#code#>
}
set {
<#code#>
}
}
@objc(propertyFromHeader9)
open var propertyFromHeader9: Int32 {
get {
<#code#>
}
set {
<#code#>
}
}
@objc(extensionMethodFromHeader2:)
open func extensionMethod(fromHeader2 param: Int32) {
<#code#>
}
```
Fixes rdar://130038221.
The recent change to make objcImpl examine class extensions has collided with a workaround some projects were employing where they declared class extension member implementations `private`. Now that we’re checking class extensions in objcImpl extensions, this workaround is creating selector conflict errors in some circumstances. Soften selector conflict errors in objcImpl extensions to warnings so the affected projects continue to build.
Fixes rdar://123347290.
