duplicates:

• SR-7380 KeyPath cannot infer type in [keyPath:] context

is duplicated by:

• SR-5973 Abbreviated key path subscript notation is ambiguous

Issue Description:

The expression "Hello"[keyPath: .count] returns an error on the key path argument: type of expression is ambiguous without more context. Both "Hello".count and "Hello"[keyPath: \String.count] work.

> swift
Welcome to Apple Swift version 4.0 (swiftlang-900.0.63.10 clang-900.0.36). Type :help for assistance.
  1> "Hello"[keyPath: \.count]
error: repl.swift:1:18: error: type of expression is ambiguous without more context
"Hello"[keyPath: \.count]
                 ^~~~~~~

  1> "Hello".count
$R0: String.IndexDistance = 5
  2> "Hello"[keyPath: \String.count]
$R1: String.IndexDistance = 5

SR-5295 is another key-path-related issue that produces the same error message, but it doesn't look like the two are related.