Add a conditional identity cast

Author: Azoy

stdlib/public/core/Builtin.swift

@@ -109,6 +109,21 @@ public func _identityCast<T, U>(_ x: T, to expectedType: U.Type) -> U {
   return Builtin.reinterpretCast(x)
 }
 
+/// Returns `x` as its concrete type `U`.
+///
+/// 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? {
+  guard T.self == U.self else {
+    return nil
+  }
+
+  return Builtin.reinterpretCast(x)
+}
+
 /// `unsafeBitCast` something to `AnyObject`.
 @usableFromInline @_transparent
 internal func _reinterpretCastToAnyObject<T>(_ x: T) -> AnyObject {

test/stdlib/Builtins.swift

@@ -307,4 +307,23 @@ tests.test("_isConcrete") {
   expectFalse(isConcrete_false(Int.self))
 }
 
+tests.test("_conditionalIdentityCast") {
+  func something<T>(with x: some Collection<T>) -> Int {
+    if let y = _conditionalIdentityCast(x, to: [Int].self) {
+      return y[0]
+    } else {
+      return 1234567890
+    }
+  }
+
+  let x = [0987654321, 1, 2]
+  expectEqual(something(with: x), 0987654321)
+
+  let y = CollectionOfOne<String>("hello world")
+  expectEqual(something(with: y), 1234567890)
+
+  let z: Any = [0, 1, 2, 3]
+  expectNil(_conditionalIdentityCast(z, to: [Int].self))
+}
+
 runAllTests()