[wpiutil] Add std::expected shim (#6310)

Its tests use Catch2 instead of GoogleTest, so we can't import them.

wpiutil/src/test/native/cpp/expected/ExpectedTest.cpp

@@ -0,0 +1,58 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include <initializer_list>
+#include <tuple>
+#include <vector>
+
+#include <gtest/gtest.h>
+
+#include "wpi/expected"
+
+namespace {
+struct TakesInitAndVariadic {
+  std::vector<int> v;
+  std::tuple<int, int> t;
+
+  template <class... Args>
+  TakesInitAndVariadic(std::initializer_list<int> l,  // NOLINT
+                       Args&&... args)
+      : v(l), t(std::forward<Args>(args)...) {}
+};
+}  // namespace
+
+TEST(ExpectedTest, Emplace) {
+  {
+    wpi::expected<std::unique_ptr<int>, int> e;
+    e.emplace(new int{42});
+    EXPECT_TRUE(e);
+    EXPECT_EQ(**e, 42);
+  }
+
+  {
+    wpi::expected<std::vector<int>, int> e;
+    e.emplace({0, 1});
+    EXPECT_TRUE(e);
+    EXPECT_EQ((*e)[0], 0);
+    EXPECT_EQ((*e)[1], 1);
+  }
+
+  {
+    wpi::expected<std::tuple<int, int>, int> e;
+    e.emplace(2, 3);
+    EXPECT_TRUE(e);
+    EXPECT_EQ(std::get<0>(*e), 2);
+    EXPECT_EQ(std::get<1>(*e), 3);
+  }
+
+  {
+    wpi::expected<TakesInitAndVariadic, int> e = wpi::make_unexpected(0);
+    e.emplace({0, 1}, 2, 3);
+    EXPECT_TRUE(e);
+    EXPECT_EQ(e->v[0], 0);
+    EXPECT_EQ(e->v[1], 1);
+    EXPECT_EQ(std::get<0>(e->t), 2);
+    EXPECT_EQ(std::get<1>(e->t), 3);
+  }
+}