[ADT] Allow std::nullopt Default in TypeSwitch

Author: kuhar

llvm/include/llvm/ADT/TypeSwitch.h

@@ -111,13 +111,22 @@ class TypeSwitch : public detail::TypeSwitchBase<TypeSwitch<T, ResultT>, T> {
       return std::move(*result);
     return defaultFn(this->value);
   }
+
   /// As a default, return the given value.
   [[nodiscard]] ResultT Default(ResultT defaultResult) {
     if (result)
       return std::move(*result);
     return defaultResult;
   }
 
+  /// Default for optional results types that accept `std::nullopt`.
+  template <typename ArgT = ResultT,
+            typename =
+                std::enable_if_t<std::is_constructible_v<ArgT, std::nullopt_t>>>
+  [[nodiscard]] ResultT Default(std::nullopt_t) {
+    return Default(ResultT(std::nullopt));
+  }
+
   /// Declare default as unreachable, making sure that all cases were handled.
   [[nodiscard]] ResultT DefaultUnreachable(
       const char *message = "Fell off the end of a type-switch") {

llvm/unittests/ADT/TypeSwitchTest.cpp

@@ -142,3 +142,13 @@ TEST(TypeSwitchTest, DefaultUnreachableWithVoid) {
   EXPECT_DEATH((void)translate(DerivedD()), "Unhandled type");
 #endif
 }
+
+TEST(TypeSwitchTest, DefaultNullopt) {
+  auto translate = [](auto value) {
+    return TypeSwitch<Base *, std::optional<int>>(&value)
+        .Case([](DerivedA *) { return 0; })
+        .Default(std::nullopt);
+  };
+  EXPECT_EQ(0, translate(DerivedA()));
+  EXPECT_EQ(std::nullopt, translate(DerivedD()));
+}