[SR-13816] FlattenSequence should conform to LazySequenceProtocol when Base does

[swift-ci]

Previous ID	SR-13816
Radar	rdar://problem/70998651
Original Reporter	laclouis5 (JIRA User)
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Improvement
Assignee	None
Priority	Medium

md5: 428272cf422670a09b2e579ae777d8bf

Issue Description:

Currently FlattenSequence does not conditionally conform to LazySequenceProtocol when its base does, however, this behavior is supported through this implementation in the standard library:

extension LazySequenceProtocol where Element: Sequence {
  /// Returns a lazy sequence that concatenates the elements of this sequence of
  /// sequences.
  @inlinable // lazy-performance
  public __consuming func joined() -> LazySequence<FlattenSequence<Elements>> {
    return FlattenSequence(_base: elements).lazy
  }
}

This is an uncommon and convoluted way to add lazy behavior to FlattenSequence (when base does), the classic approach in the standard library is the following one:

extension Slice: LazySequenceProtocol where Base: LazySequenceProtocol { }
extension ReversedCollection: LazySequenceProtocol where Base: LazySequenceProtocol { }

This minor issue may feel awkward because the returning type of someSequenceOfSequence.lazy.joined() is not FlattenSequence<LazySequence<...>> as expected but LazySequence<FlattenSequence<...>>.
Fixing this would reduce the number of overloads of the joined() method to 1 and make the standard library more consistent.

[typesanitizer]

@swift-ci create