Add a template gadget to detect whether a type supports the ostream operator.

I.e. for a type `Example`, if
```
std::ostream& operator<<(std::ostream&, const Example&);
```
is declared, `absl::HasOstreamOperator<Example>::value` is `true`.
PiperOrigin-RevId: 574637891
Change-Id: I123d8f35a6e3ea894745133f7d81b1610fb475d5

1 files changed, 41 insertions, 0 deletions

diff --git a/absl/strings/has_ostream_operator_test.cc b/absl/strings/has_ostream_operator_test.cc
new file mode 100644
index 00000000..9ef41366
--- /dev/null
+++ b/absl/strings/has_ostream_operator_test.cc
@@ -0,0 +1,41 @@
+// Copyright 2023 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/has_ostream_operator.h"
+
+#include <ostream>
+#include <string>
+
+#include "gtest/gtest.h"
+#include "absl/types/optional.h"
+
+namespace {
+
+struct TypeWithoutOstreamOp {};
+
+struct TypeWithOstreamOp {
+  friend std::ostream& operator<<(std::ostream& os, const TypeWithOstreamOp&) {
+    return os;
+  }
+};
+
+TEST(HasOstreamOperatorTest, Works) {
+  EXPECT_TRUE(absl::HasOstreamOperator<int>::value);
+  EXPECT_TRUE(absl::HasOstreamOperator<std::string>::value);
+  EXPECT_FALSE(absl::HasOstreamOperator<absl::optional<int>>::value);
+  EXPECT_FALSE(absl::HasOstreamOperator<TypeWithoutOstreamOp>::value);
+  EXPECT_TRUE(absl::HasOstreamOperator<TypeWithOstreamOp>::value);
+}
+
+}  // namespace