Issue Description:

I can't quite figure out what it is about this code that produces this result, but given this input:

import Foundation // for components(separatedBy:)
enum Example: String { case whatever }
let impliedArray = "x".components(separatedBy: " ")
let dict: [String: Codable] = [
    "key1": impliedArray[0],
    "key2": Example.whatever,
].merging(impliedArray.map { ($0, $0) }, uniquingKeysWith: { $1 })

The compiler gives this misleading error:

error: repl.swift:6:21: error: cannot convert value of type 'Example' to expected dictionary value type 'String'
    "key2": Example.whatever,
                    ^

If impliedArray is removed from either of the usage sites (such as replacing it with a literal like ["x"]), the error is much more what one would expect:

error: repl.swift:6:21: error: cannot convert value of type 'Example' to expected dictionary value type 'Codable' (aka 'Decodable & Encodable')
    "key2": Example.whatever,
                    ^

In both cases, adding Codable conformance to the enum immediately silences the error and compilation will succeed.

This appears to require, at the least:

• At least one of the dictionary's values to come from a variable of inferred array or dictionary type.

• The input sequence for .merging(_:uniquingKeysWith: ) also come from a variable of inferred array or dictionary type.

The Example type given here can be just about anything not already Codable; for instance, the same issue appears if the declaration is:

struct Example { let x = 0 }