[SR-7082] Reconsider prohibition on overriding a property with a stored property

[jckarter]

Previous ID	SR-7082
Radar	None
Original Reporter	@jckarter
Type	Bug

Additional Detail from JIRA

Votes	2
Component/s	Compiler
Labels	Bug, LanguageFeatureRequest
Assignee	None
Priority	Medium

md5: 400c319da94f2aeb9aecf260173023c7

Issue Description:

We won't let you override a property from a base class with a stored property:

class Base {
  var property: Int { fatalError("abstract") }
}

class Derived: Base {
  override var property: Int // an error
}

This prohibition came from fear that people might override a property with new storage by accidentally redeclaring the property in their derived class. This decision was made in ancient times before the override keyword was even added and was never revisited. We should reconsider it now that overrides are always explicit.

One remaining potential ambiguity is in the behavior of overrides with observers. Currently, an override with observers always adds the observers on top of the super implementation. If we allow stored properties to be overrides, it may be ambiguous whether an override with didSet/willSet is intended to be a new stored property with and observers or an extension of the base class property with observers.

[belkadan]

Any change here would have to go through the Swift Evolution Process, of course.

[mattneub]

Along the same lines (I think), it remains surprising (I think) that an initialized `let` property cannot be overridden by another initialized `let` property. It's hard to see what harm that could do.