Parse: Parse an explicit 'nil' default argument as DefaultArgumentKind::NilLiteral

We support a special DefaultArgumentKind::NilLiteral, but it was only
used for synthesized and imported default arguments as well.

Let's also use it for parsed default arguments that are exactly 'nil',
so that rethrows checking can special-case them.

Author: slavapestov

lib/Parse/ParsePattern.cpp

@@ -39,6 +39,12 @@ static DefaultArgumentKind getDefaultArgKind(Expr *init) {
   if (!init)
     return DefaultArgumentKind::None;
 
+  // Parse an as-written 'nil' expression as the special NilLiteral kind,
+  // which is emitted by the caller and can participate in rethrows
+  // checking.
+  if (isa<NilLiteralExpr>(init))
+    return DefaultArgumentKind::NilLiteral;
+
   auto magic = dyn_cast<MagicIdentifierLiteralExpr>(init);
   if (!magic)
     return DefaultArgumentKind::Normal;

test/SILGen/constrained_extensions.swift

@@ -44,7 +44,7 @@ extension Array where Element == Int {
 
   // CHECK-LABEL: sil non_abi [serialized] [ossa] @$sSa22constrained_extensionsSiRszlE12staticMethod1eS2iSg_tFZfA_ : $@convention(thin) () -> Optional<Int>
   // CHECK-LABEL: sil [ossa] @$sSa22constrained_extensionsSiRszlE12staticMethod1eS2iSg_tFZ : $@convention(method) (Optional<Int>, @thin Array<Int>.Type) -> Int
-  public static func staticMethod(e: Element? = nil) -> Element {
+  public static func staticMethod(e: Element? = (nil)) -> Element {
     return e!
   }
 
@@ -104,7 +104,7 @@ extension Dictionary where Key == Int {
   // CHECK-LABEL: sil non_abi [serialized] [ossa] @$sSD22constrained_extensionsSiRszrlE12staticMethod1k1vq_SiSg_q_SgtFZfA_ : $@convention(thin) <Key, Value where Key == Int> () -> Optional<Int>
   // CHECK-LABEL: sil non_abi [serialized] [ossa] @$sSD22constrained_extensionsSiRszrlE12staticMethod1k1vq_SiSg_q_SgtFZfA0_ : $@convention(thin) <Key, Value where Key == Int> () -> @out Optional<Value>
   // CHECK-LABEL: sil [ossa] @$sSD22constrained_extensionsSiRszrlE12staticMethod1k1vq_SiSg_q_SgtFZ : $@convention(method) <Key, Value where Key == Int> (Optional<Int>, @in_guaranteed Optional<Value>, @thin Dictionary<Int, Value>.Type) -> @out Value
-  public static func staticMethod(k: Key? = nil, v: Value? = nil) -> Value {
+  public static func staticMethod(k: Key? = (nil), v: Value? = (nil)) -> Value {
     return v!
   }
 
@@ -191,7 +191,7 @@ extension Array where Element == Int {
 
     // CHECK-LABEL: sil hidden [ossa] @$sSa22constrained_extensionsSiRszlE6NestedV10hasDefault1eySiSg_tFfA_ : $@convention(thin) () -> Optional<Int>
     // CHECK-LABEL: sil hidden [ossa] @$sSa22constrained_extensionsSiRszlE6NestedV10hasDefault1eySiSg_tF : $@convention(method) (Optional<Int>, @inout Array<Int>.Nested) -> ()
-    mutating func hasDefault(e: Element? = nil) {
+    mutating func hasDefault(e: Element? = (nil)) {
       self.e = e
     }
   }

test/SILGen/default_arguments.swift

@@ -284,7 +284,7 @@ func test_r18400194() {
 //   Don't add capture arguments to local default argument generators.
 func localFunctionWithDefaultArg() {
   var z = 5
-  func bar(_ x: Int? = nil) {
+  func bar(_ x: Int? = (nil)) {
     z += 1
   }
   bar()

test/SILGen/default_arguments_inherited.swift

@@ -7,7 +7,7 @@
 // derived class generic signature.
 
 class Puppy<T, U> {
-  init(t: T? = nil, u: U? = nil) {}
+  init(t: T? = (nil), u: U? = (nil)) {}
 }
 
 class Chipmunk : Puppy<Int, String> {}

test/SILGen/default_arguments_nil.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+public func takesAnInt(_: Int? = nil) {}
+
+// CHECK-LABEL: sil [ossa] @$s21default_arguments_nil15callsTakesAnIntyyF : $@convention(thin) () -> () {
+// CHECK: [[NIL:%.*]] = enum $Optional<Int>, #Optional.none!enumelt
+// CHECK: [[FN:%.*]] = function_ref @$s21default_arguments_nil10takesAnIntyySiSgF
+// CHECK: apply [[FN]]([[NIL]]) : $@convention(thin) (Optional<Int>) -> ()
+// CHECK: return
+public func callsTakesAnInt() {
+  takesAnInt()
+}