Add Result::unwrap_or_default

Author: danakj

subspace/result/result.h

@@ -666,6 +666,28 @@ class [[nodiscard]] Result final {
     }
   }
 
+  /// Returns the contained Ok value or a default.
+  ///
+  /// Consumes the Result and, if it held an Ok value, the value is returned.
+  /// Otherwise the default value of the Ok value's type is returned.
+  constexpr T unwrap_or_default() && noexcept
+    requires(!std::is_reference_v<T> &&
+             (std::is_void_v<T> || ::sus::construct::Default<T>))
+  {
+    ResultState was = ::sus::mem::replace(mref(state_), ResultState::IsMoved);
+    check_with_message(
+        was != ResultState::IsMoved,
+        *"called `Result::unwrap_or_default()` on a moved Result");
+    if constexpr (!std::is_void_v<T>) {
+      if (was == ResultState::IsOk) {
+        return ::sus::mem::take_and_destruct(::sus::marker::unsafe_fn,
+                                             mref(storage_.ok_));
+      } else {
+        return T();
+      }
+    }
+  }
+
   /// Returns the contained `Ok` value, consuming the self value, without
   /// checking that the value is not an `Err`.
   ///

subspace/result/result_unittest.cc

@@ -572,6 +572,31 @@ TEST(Result, Unwrap) {
   EXPECT_EQ(&cu, &m);
 }
 
+TEST(Result, UnwrapOrDefault) {
+  {
+    constexpr auto a = Result<i32, Error>::with(3_i32).unwrap_or_default();
+    static_assert(std::same_as<decltype(a), const i32>);
+    EXPECT_EQ(a, 3_i32);
+
+    constexpr auto d =
+        Result<i32, Error>::with_err(Error()).unwrap_or_default();
+    static_assert(std::same_as<decltype(d), const i32>);
+    EXPECT_EQ(d, 0_i32);
+  }
+  {
+    // Returns void, doesn't panic.
+    Result<void, Error>::with().unwrap_or_default();
+    static_assert(
+        std::same_as<decltype(Result<void, Error>::with().unwrap_or_default()),
+                     void>);
+    // Returns void, doesn't panic.
+    Result<void, Error>::with_err(Error()).unwrap_or_default();
+    static_assert(
+        std::same_as<decltype(Result<void, Error>::with().unwrap_or_default()),
+                     void>);
+  }
+}
+
 TEST(ResultDeathTest, UnwrapWithErr) {
 #if GTEST_HAS_DEATH_TEST
   auto r = Result<i32, Error>::with_err(Error());