[stdlib] Prioritizing non-failing String.init_:String)

Author: Max Moiseev

stdlib/public/core/String.swift

@@ -78,6 +78,12 @@ public protocol StringProtocol
   ) rethrows -> Result
 }
 
+extension StringProtocol /* : LosslessStringConvertible */ {
+  public init?(_ description: String) {
+    self.init(description.characters)
+  }
+}
+
 /// Call body with a pointer to zero-terminated sequence of
 /// `TargetEncoding.CodeUnit` representing the same string as `source`, when
 /// `source` is interpreted as being encoded with `SourceEncoding`.
@@ -1023,12 +1029,6 @@ extension String : CustomStringConvertible {
   }
 }
 
-extension String : LosslessStringConvertible {
-  public init?(_ description: String) {
-    self = description
-  }
-}
-
 extension String {
   @available(*, unavailable, renamed: "append(_:)")
   public mutating func appendContentsOf(_ other: String) {

stdlib/public/core/StringRangeReplaceableCollection.swift.gyb

@@ -41,8 +41,8 @@ extension String : StringProtocol {
   // - init<S>(_ characters: S) where S : Sequence, S.Iterator.Element == Character
   // Cannot simply do init(_: String) as that would itself be ambiguous with
   // init?(_ description: String)
-  public init<T : StringProtocol>(_ other: T) {
-    self.init(other.characters)
+  public init(_ other: String) {
+    self.init(other._core)
   }
 
   /// The position of the first character in a nonempty string.

test/stdlib/StringAPI.swift

@@ -362,9 +362,12 @@ StringTests.test("CompareStringsWithUnpairedSurrogates")
 }
 
 StringTests.test("String.init(_:String)") {
-  let _: String = String("" as String) // should compile without ambiguities
+  var s = String("")
+  expectType(String.self, &s)
+  var _ = String("") as String? // should also compile, but not be the default
 }
 
+
 StringTests.test("[String].joined() -> String") {
   let s = ["hello", "world"].joined()
   _ = s == "" // should compile without error