[SR-11130] Type sugar supports nested types inconsistently

[beccadax]

Previous ID	SR-11130
Radar	None
Original Reporter	@beccadax
Type	Bug

Environment

True in at least Swift 5.0 and 5.1.

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 1 | |Component/s | Compiler | |Labels | Bug, Parser | |Assignee | None | |Priority | Medium | md5: 2ff725049d085d988e443dcc166c5a65

duplicates:

• SR-5108 Collection Type Shorthand Syntax Issues

is duplicated by:

• SR-11196 Cannot Reference 'Dictionary.Index' with Dictionary Shorthand Syntax

Issue Description:

This is allowed:

[String: Int].Keys.self

This fails to parse:

let keys: [String: Int].Keys

Array behaves analogously.

This presumably happens because types in expressions are resolved by special typechecker preprocessing, but these should probably be treated consistently. However, removing the expression form would be source-breaking, and adding the declaration form might be considered a language change requiring Evolution (if it's not an outright bug), so neither solution is completely trivial. Core team members have said that the lack of support in declarations is a bug that can be fixed without an Evolution proposal, but please note the change in the CHANGELOG file.

[belkadan]

I'd probably consider it a bug that the type syntax doesn't work, but @DougGregor or @jckarter would have to confirm as Core Team members.

[jckarter]

I agree that's a bug.