Create Error type (#1137)

Co-authored-by: Geoff Romer <gromer@google.com>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

common/error_test.cpp

@@ -0,0 +1,47 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "common/error.h"
+
+#include <gtest/gtest.h>
+
+namespace Carbon::Testing {
+namespace {
+
+TEST(ErrorTest, Error) {
+  Error err("test");
+  EXPECT_EQ(err.message(), "test");
+}
+
+TEST(ErrorTest, ErrorEmptyString) {
+  ASSERT_DEATH({ Error err(""); }, "CHECK failure at");
+}
+
+auto IndirectError() -> Error { return Error("test"); }
+
+TEST(ErrorTest, IndirectError) { EXPECT_EQ(IndirectError().message(), "test"); }
+
+TEST(ErrorTest, ErrorOr) {
+  ErrorOr<int> err(Error("test"));
+  EXPECT_FALSE(err.ok());
+  EXPECT_EQ(err.error().message(), "test");
+}
+
+TEST(ErrorTest, ErrorOrValue) { EXPECT_TRUE(ErrorOr<int>(0).ok()); }
+
+auto IndirectErrorOrTest() -> ErrorOr<int> { return Error("test"); }
+
+TEST(ErrorTest, IndirectErrorOr) { EXPECT_FALSE(IndirectErrorOrTest().ok()); }
+
+struct Val {
+  int val;
+};
+
+TEST(ErrorTest, ErrorOrArrowOp) {
+  ErrorOr<Val> err({1});
+  EXPECT_EQ(err->val, 1);
+}
+
+}  // namespace
+}  // namespace Carbon::Testing