Add unittest for covering ErrorBuilder implicit cast and metaprogramming (#2882)

- update unittest of Error, cover the casting of ErrorBuilder.
- add unittest of metaprogramming.h

common/BUILD

@@ -115,6 +115,17 @@ cc_library(
     hdrs = ["metaprogramming.h"],
 )
 
+cc_test(
+    name = "metaprogramming_test",
+    srcs = ["metaprogramming_test.cpp"],
+    deps = [
+        ":metaprogramming",
+        "//common:gtest_main",
+        "@com_google_googletest//:gtest",
+        "@llvm-project//llvm:Support",
+    ],
+)
+
 cc_library(
     name = "ostream",
     hdrs = ["ostream.h"],

common/error_test.cpp

@@ -97,5 +97,17 @@ TEST(ErrorTest, AssignOrReturnHasErrorInExpected) {
   EXPECT_EQ(result.error().message(), "error");
 }
 
+TEST(ErrorTest, ErrorBuilderOperatorImplicitCast) {
+  ErrorOr<int> result1 = ErrorBuilder() << "msg";
+  ASSERT_FALSE(result1.ok());
+  EXPECT_EQ(result1.error().message(), "msg");
+
+  auto result2 = static_cast<Error>(ErrorBuilder("TestFunc") << "msg");
+  std::string result2_output;
+  llvm::raw_string_ostream oss(result2_output);
+  result2.Print(oss);
+  EXPECT_EQ(oss.str(), "TestFunc: msg");
+}
+
 }  // namespace
 }  // namespace Carbon::Testing

common/metaprogramming_test.cpp

@@ -0,0 +1,32 @@
+// 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/metaprogramming.h"
+
+#include <gtest/gtest.h>
+
+#include <string>
+
+#include "llvm/Support/raw_ostream.h"
+
+namespace Carbon::Testing {
+namespace {
+
+TEST(MetaProgrammingTest, RequiresBasic) {
+  bool result = Requires<int, int>([](int a, int b) { return a + b; });
+  EXPECT_TRUE(result);
+}
+
+struct TypeWithPrint {
+  void Print(llvm::raw_ostream& os) const { os << "Test"; }
+};
+
+TEST(MetaProgrammingTest, RequiresPrintMethod) {
+  bool result = Requires<const TypeWithPrint, llvm::raw_ostream>(
+      [](auto&& t, auto&& out) -> decltype(t.Print(out)) {});
+  EXPECT_TRUE(result);
+}
+
+}  // namespace
+}  // namespace Carbon::Testing