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

CMake/AbseilDll.cmake

@@ -307,6 +307,7 @@ set(ABSL_INTERNAL_DLL_FILES
   "strings/internal/stringify_sink.h"
   "strings/internal/stringify_sink.cc"
   "strings/has_absl_stringify.h"
+  "strings/has_ostream_operator.h"
   "strings/match.cc"
   "strings/match.h"
   "strings/numbers.cc"

absl/strings/BUILD.bazel

@@ -194,6 +194,31 @@ cc_test(
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
+        "//absl/types:optional",
+        "@com_google_googletest//:gtest",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "has_ostream_operator",
+    hdrs = ["has_ostream_operator.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "has_ostream_operator_test",
+    size = "small",
+    srcs = ["has_ostream_operator_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":has_ostream_operator",
+        "//absl/types:optional",
         "@com_google_googletest//:gtest",
         "@com_google_googletest//:gtest_main",
     ],

absl/strings/CMakeLists.txt

@@ -103,6 +103,18 @@ absl_cc_library(
   PUBLIC
 )
 
+absl_cc_library(
+  NAME
+    has_ostream_operator
+  HDRS
+    "has_ostream_operator.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+  PUBLIC
+)
+
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
@@ -162,12 +174,26 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::optional
     absl::strings
     GTest::gmock_main
 )
 
 absl_cc_test(
   NAME
+    has_ostream_operator_test
+  SRCS
+    "has_ostream_operator_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::has_ostream_operator
+    absl::optional
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
     ascii_test
   SRCS
     "ascii_test.cc"

absl/strings/has_absl_stringify_test.cc

@@ -17,6 +17,7 @@
 #include <string>
 
 #include "gtest/gtest.h"
+#include "absl/types/optional.h"
 
 namespace {
 
@@ -32,6 +33,8 @@ TEST(HasAbslStringifyTest, Works) {
   EXPECT_FALSE(absl::HasAbslStringify<std::string>::value);
   EXPECT_FALSE(absl::HasAbslStringify<TypeWithoutAbslStringify>::value);
   EXPECT_TRUE(absl::HasAbslStringify<TypeWithAbslStringify>::value);
+  EXPECT_FALSE(
+      absl::HasAbslStringify<absl::optional<TypeWithAbslStringify>>::value);
 }
 
 }  // namespace

absl/strings/has_ostream_operator.h

+// 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.
+
+#ifndef ABSL_STRINGS_HAS_OSTREAM_OPERATOR_H_
+#define ABSL_STRINGS_HAS_OSTREAM_OPERATOR_H_
+
+#include <ostream>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Detects if type `T` supports streaming to `std::ostream`s with `operator<<`.
+
+template <typename T, typename = void>
+struct HasOstreamOperator : std::false_type {};
+
+template <typename T>
+struct HasOstreamOperator<
+    T, std::enable_if_t<std::is_same<
+           std::ostream&, decltype(std::declval<std::ostream&>()
+                                   << std::declval<const T&>())>::value>>
+    : std::true_type {};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_HAS_OSTREAM_OPERATOR_H_

absl/strings/has_ostream_operator_test.cc

+// 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