Support TryDefault for std::optional<T>

When T satisfies Default.

Author: danakj

sus/option/compat_option.h

@@ -150,6 +150,14 @@ struct sus::ops::TryImpl<std::optional<T>> {
   constexpr static std::optional<T> from_output(Output t) {
     return std::optional<T>(std::in_place, ::sus::move(t));
   }
+
+  // Implements sus::ops::TryDefault for `std::optional<T>` if `T` satisfies
+  // `Default`.
+  constexpr static std::optional<T> from_default() noexcept
+    requires(sus::construct::Default<T>)
+  {
+    return std::optional<T>(std::in_place, T());
+  }
 };
 
 }  // namespace sus

sus/option/compat_option_unittest.cc

@@ -269,8 +269,11 @@ TEST(CompatOption, FromOptionalCopyWithConversion) {
 
 TEST(CompatOption, Try) {
   static_assert(sus::ops::Try<std::optional<i32>>);
-  EXPECT_EQ(sus::ops::try_is_success(std::optional<i32>(std::in_place, 1)), true);
-  EXPECT_EQ(sus::ops::try_is_success(std::optional<i32>()), false);
+  static_assert(sus::ops::TryDefault<std::optional<i32>>);
+  static_assert(sus::ops::try_is_success(std::optional<i32>(std::in_place, 1)));
+  static_assert(!sus::ops::try_is_success(std::optional<i32>()));
+  static_assert(sus::ops::try_from_default<std::optional<i32>>() ==
+                std::optional<i32>(0_i32));
 }
 
 }  // namespace