Rename to _specialize

Author: Azoy

stdlib/public/core/Builtin.swift

@@ -109,14 +109,13 @@ public func _identityCast<T, U>(_ x: T, to expectedType: U.Type) -> U {
   return Builtin.reinterpretCast(x)
 }
 
-/// Returns `x` as its concrete type `U`.
+/// Returns `x` as its concrete type `U`, or `nil` if `x` has a different
+/// concrete type.
 ///
 /// This cast can be useful for dispatching to specializations of generic
 /// functions.
-///
-/// - Requires: `x` has type `U`.
 @_alwaysEmitIntoClient
-public func _conditionalIdentityCast<T, U>(_ x: T, to: U.Type) -> U? {
+public func _specialize<T, U>(_ x: T, for: U.Type) -> U? {
   guard T.self == U.self else {
     return nil
   }

test/stdlib/Builtins.swift

@@ -307,9 +307,9 @@ tests.test("_isConcrete") {
   expectFalse(isConcrete_false(Int.self))
 }
 
-tests.test("_conditionalIdentityCast") {
+tests.test("_specialize") {
   func something<T>(with x: some Collection<T>) -> Int {
-    if let y = _conditionalIdentityCast(x, to: [Int].self) {
+    if let y = _specialize(x, for: [Int].self) {
       return y[0]
     } else {
       return 1234567890
@@ -323,7 +323,7 @@ tests.test("_conditionalIdentityCast") {
   expectEqual(something(with: y), 1234567890)
 
   let z: Any = [0, 1, 2, 3]
-  expectNil(_conditionalIdentityCast(z, to: [Int].self))
+  expectNil(_specialize(z, for: [Int].self))
 }
 
 runAllTests()