[Version 12.0] Feature support for collection expressions

[RexJaeschke]

This is Rex's adaptation of the corresponding MS proposal.

• Some sections of the MS spec appears to be targeted to the implementer:

  • The essence of the “Ref safety” section has been reduced to a single paragraph at the end of “Implicit collection expression conversions.”
  • Nothing was adopted from the “Extension methods” section.
  • Nothing was adopted from the “Span types” section.
  • Nothing was adopted from the long/detailed “Collection literal translation” section.
  • Nothing was adopted from the “Unsupported scenarios” section.
  • Nothing was adopted from the “Resolved questions” section.

• Syntax ambiguities: It’s not clear what the final situation is; nothing was added from this section.

• It isn’t clear what might/should be adopted from the “Unresolved questions” section.