NSDate(timeIntervalSince1970: 0) == 0 がコンパイルエラーにならない件

command

$ swiftc -emit-silgen -sdk $(xcrun --show-sdk-path --sdk macosx) date.swift > date.sil

date.sil

sil_stage raw

import Builtin
import Swift
import SwiftShims

// date.b : Swift.Bool
sil_global [let] @_Tv4date1bSb : $Bool

// main
sil @main : $@convention(c) (Int32, UnsafeMutablePointer<UnsafeMutablePointer<Int8>>) -> Int32 {
bb0(%0 : $Int32, %1 : $UnsafeMutablePointer<UnsafeMutablePointer<Int8>>):
  // function_ref Swift._didEnterMain (Swift.Int32, argv : Swift.UnsafeMutablePointer<Swift.UnsafeMutablePointer<Swift.Int8>>) -> ()
  %2 = function_ref @_TFSs13_didEnterMainFTVSs5Int324argvGVSs20UnsafeMutablePointerGS0_VSs4Int8___T_ : $@convention(thin) (Int32, UnsafeMutablePointer<UnsafeMutablePointer<Int8>>) -> () // user: %3
  %3 = apply %2(%0, %1) : $@convention(thin) (Int32, UnsafeMutablePointer<UnsafeMutablePointer<Int8>>) -> ()
  %4 = global_addr @_Tv4date1bSb : $*Bool         // user: %22
  // function_ref static ObjectiveC.== infix (__ObjC.NSObject, __ObjC.NSObject) -> Swift.Bool
  %5 = function_ref @_TZF10ObjectiveCoi2eeFTCSo8NSObjectS0__Sb : $@convention(thin) (@owned NSObject, @owned NSObject) -> Bool // user: %21
  // function_ref __ObjC.NSDate.__allocating_init (__ObjC.NSDate.Type)(timeIntervalSince1970 : Swift.Double) -> __ObjC.NSDate
  %6 = function_ref @_TFCSo6NSDateCfMS_FT21timeIntervalSince1970Sd_S_ : $@convention(thin) (Double, @thick NSDate.Type) -> @owned NSDate // user: %12
  %7 = metatype $@thick NSDate.Type               // user: %12
  // function_ref Swift.Double.init (Swift.Double.Type)(_builtinIntegerLiteral : Builtin.Int2048) -> Swift.Double
  %8 = function_ref @_TFSdCfMSdFT22_builtinIntegerLiteralBi2048__Sd : $@convention(thin) (Builtin.Int2048, @thin Double.Type) -> Double // user: %11
  %9 = metatype $@thin Double.Type                // user: %11
  %10 = integer_literal $Builtin.Int2048, 0       // user: %11
  %11 = apply %8(%10, %9) : $@convention(thin) (Builtin.Int2048, @thin Double.Type) -> Double // user: %12
  %12 = apply %6(%11, %7) : $@convention(thin) (Double, @thick NSDate.Type) -> @owned NSDate // user: %13
  %13 = upcast %12 : $NSDate to $NSObject         // user: %21
  // function_ref ext.Foundation.Swift.Int._bridgeToObjectiveC (Swift.Int)() -> __ObjC.NSNumber
  %14 = function_ref @_TFE10FoundationSi19_bridgeToObjectiveCfSiFT_CSo8NSNumber : $@convention(method) (Int) -> @owned NSNumber // user: %19
  // function_ref Swift.Int.init (Swift.Int.Type)(_builtinIntegerLiteral : Builtin.Int2048) -> Swift.Int
  %15 = function_ref @_TFSiCfMSiFT22_builtinIntegerLiteralBi2048__Si : $@convention(thin) (Builtin.Int2048, @thin Int.Type) -> Int // user: %18
  %16 = metatype $@thin Int.Type                  // user: %18
  %17 = integer_literal $Builtin.Int2048, 0       // user: %18
  %18 = apply %15(%17, %16) : $@convention(thin) (Builtin.Int2048, @thin Int.Type) -> Int // user: %19
  %19 = apply %14(%18) : $@convention(method) (Int) -> @owned NSNumber // user: %20
  %20 = upcast %19 : $NSNumber to $NSObject       // user: %21
  %21 = apply %5(%13, %20) : $@convention(thin) (@owned NSObject, @owned NSObject) -> Bool // user: %22
  store %21 to %4 : $*Bool                        // id: %22
  %23 = integer_literal $Builtin.Int32, 0         // user: %24
  %24 = struct $Int32 (%23 : $Builtin.Int32)      // user: %25
  return %24 : $Int32                             // id: %25
}

// Swift._didEnterMain (Swift.Int32, argv : Swift.UnsafeMutablePointer<Swift.UnsafeMutablePointer<Swift.Int8>>) -> ()
sil [transparent] [fragile] @_TFSs13_didEnterMainFTVSs5Int324argvGVSs20UnsafeMutablePointerGS0_VSs4Int8___T_ : $@convention(thin) (Int32, UnsafeMutablePointer<UnsafeMutablePointer<Int8>>) -> ()

// static ObjectiveC.== infix (__ObjC.NSObject, __ObjC.NSObject) -> Swift.Bool
sil @_TZF10ObjectiveCoi2eeFTCSo8NSObjectS0__Sb : $@convention(thin) (@owned NSObject, @owned NSObject) -> Bool

// __ObjC.NSDate.__allocating_init (__ObjC.NSDate.Type)(timeIntervalSince1970 : Swift.Double) -> __ObjC.NSDate
sil shared @_TFCSo6NSDateCfMS_FT21timeIntervalSince1970Sd_S_ : $@convention(thin) (Double, @thick NSDate.Type) -> @owned NSDate {
bb0(%0 : $Double, %1 : $@thick NSDate.Type):
  %2 = thick_to_objc_metatype %1 : $@thick NSDate.Type to $@objc_metatype NSDate.Type // user: %3
  %3 = alloc_ref_dynamic [objc] %2 : $@objc_metatype NSDate.Type, $NSDate // user: %5
  // function_ref @nonobjc __ObjC.NSDate.init (__ObjC.NSDate.Type)(timeIntervalSince1970 : Swift.Double) -> __ObjC.NSDate
  %4 = function_ref @_TTOFCSo6NSDatecfMS_FT21timeIntervalSince1970Sd_S_ : $@convention(method) (Double, @owned NSDate) -> @owned NSDate // user: %5
  %5 = apply %4(%0, %3) : $@convention(method) (Double, @owned NSDate) -> @owned NSDate // user: %6
  return %5 : $NSDate                             // id: %6
}

// Swift.Double.init (Swift.Double.Type)(_builtinIntegerLiteral : Builtin.Int2048) -> Swift.Double
sil [transparent] [fragile] @_TFSdCfMSdFT22_builtinIntegerLiteralBi2048__Sd : $@convention(thin) (Builtin.Int2048, @thin Double.Type) -> Double

// ext.Foundation.Swift.Int._bridgeToObjectiveC (Swift.Int)() -> __ObjC.NSNumber
sil [_semantics "convertToObjectiveC"] @_TFE10FoundationSi19_bridgeToObjectiveCfSiFT_CSo8NSNumber : $@convention(method) (Int) -> @owned NSNumber

// Swift.Int.init (Swift.Int.Type)(_builtinIntegerLiteral : Builtin.Int2048) -> Swift.Int
sil [transparent] [fragile] @_TFSiCfMSiFT22_builtinIntegerLiteralBi2048__Si : $@convention(thin) (Builtin.Int2048, @thin Int.Type) -> Int

// @nonobjc __ObjC.NSDate.init (__ObjC.NSDate.Type)(timeIntervalSince1970 : Swift.Double) -> __ObjC.NSDate
sil shared @_TTOFCSo6NSDatecfMS_FT21timeIntervalSince1970Sd_S_ : $@convention(method) (Double, @owned NSDate) -> @owned NSDate {
bb0(%0 : $Double, %1 : $NSDate):
  %2 = class_method [volatile] %1 : $NSDate, #NSDate.init!initializer.1.foreign : NSDate.Type -> (timeIntervalSince1970: Double) -> NSDate , $@convention(objc_method) (Double, @owned NSDate) -> @owned NSDate // user: %3
  %3 = apply %2(%0, %1) : $@convention(objc_method) (Double, @owned NSDate) -> @owned NSDate // user: %4
  return %3 : $NSDate                             // id: %4
}

date.swift

import Foundation

let b: Bool = NSDate(timeIntervalSince1970: 0) == 0

結論

0がNSNumberに変換されて

@warn_unused_result
public func ==(lhs: NSObject, rhs: NSObject) -> Bool

に適用される。