swift234

[Type system] Infer 'Any' for array elements and dictionary values and 'AnyHashable' for dictionary keys.

The id-as-Any work regressed cases where Swift code could specify
heterogeneous collection literals, e.g.,

    var states: [String: Any] = [
      "California": [
        "population": 37_000_000,
        "cities": ["Los Angeles", "San Diego", "San Jose"],
      ],
      "Oregon": [
        "population": 4_000_000,
        "cities": ["Portland", "Salem", "Eugene"],
      ]
    ]

Prior to this, the code worked (when Foundation was imported) because
we'd end up with literals of type [NSObject : AnyObject].

The new defaulting rule says that the element type of an array literal
and the key/value types of a dictionary literal can be defaulted if no
stronger type can be inferred. The default type is:

  Any, for the element type of an array literal or the value type of a
  dictionary literal, or

  AnyHashable, for the key type of a dictionary literal.

The latter is intended to compose with implicit conversions to
AnyHashable, so the most-general inferred dictionary type is
[AnyHashable : Any] and will work for any plausible dictionary
literal.

To prevent this inference from diluting types too greatly, we don't
allow this inference in "top-level" expressions, e.g.,

  let d = ["a" : 1, "b" : "two"]

will produce an error because it's a heterogeneous dictionary literal
at the top level. One should annotate this with, e.g.,

  let d = ["a" : 1, "b" : "two"] as [String : Any]

However, we do permit heterogeneous collections in nested positions,
to support cases like the original motivating example.

Fixes rdar://problem/27661580.