`absl::Overload()` which returns a functor that provides overloads based on the functors passed to it.

PiperOrigin-RevId: 568476251
Change-Id: Ic625c9b5300d1db496979c178ca1e655581f9276

4 files changed, 255 insertions, 0 deletions

diff --git a/absl/functional/BUILD.bazel b/absl/functional/BUILD.bazel
index 4ceac539..28718257 100644
--- a/absl/functional/BUILD.bazel
+++ b/absl/functional/BUILD.bazel
@@ -113,6 +113,32 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "overload",
+    hdrs = ["overload.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/meta:type_traits",
+    ],
+)
+
+cc_test(
+    name = "overload_test",
+    size = "small",
+    srcs = ["overload_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":overload",
+        "//absl/base:config",
+        "//absl/strings",
+        "//absl/strings:string_view",
+        "//absl/types:variant",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "function_type_benchmark",
     srcs = [
diff --git a/absl/functional/CMakeLists.txt b/absl/functional/CMakeLists.txt
index c704e049..602829cb 100644
--- a/absl/functional/CMakeLists.txt
+++ b/absl/functional/CMakeLists.txt
@@ -108,3 +108,27 @@ absl_cc_test(
     absl::test_instance_tracker
     GTest::gmock_main
 )
+
+absl_cc_library(
+  NAME
+    overload
+  HDRS
+    "overload.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::meta
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    overload_test
+  SRCS
+    "overload_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::strings
+    GTest::gmock_main
+)
diff --git a/absl/functional/overload.h b/absl/functional/overload.h
new file mode 100644
index 00000000..4651f14b
--- /dev/null
+++ b/absl/functional/overload.h
@@ -0,0 +1,75 @@
+// 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.
+//
+// -----------------------------------------------------------------------------
+// File: overload.h
+// -----------------------------------------------------------------------------
+//
+// `absl::Overload()` returns a functor that provides overloads based on the
+// functors passed to it.
+// Before using this function, consider whether named function overloads would
+// be a better design.
+// One use case for this is locally defining visitors for `std::visit` inside a
+// function using lambdas.
+
+// Example: Using  `absl::Overload` to define a visitor for `std::variant`.
+//
+// std::variant<int, std::string, double> v(int{1});
+//
+// assert(std::visit(absl::Overload(
+//                        [](int) -> absl::string_view { return "int"; },
+//                        [](const std::string&) -> absl::string_view {
+//                          return "string";
+//                        },
+//                        [](double) -> absl::string_view { return "double"; }),
+//                     v) == "int");
+//
+// Note: This requires C++17.
+
+#ifndef ABSL_FUNCTIONAL_OVERLOAD_H_
+#define ABSL_FUNCTIONAL_OVERLOAD_H_
+
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
+
+template <int&... ExplicitArgumentBarrier, typename... T>
+auto Overload(T&&... ts) {
+  struct OverloadImpl : absl::remove_cvref_t<T>... {
+    using absl::remove_cvref_t<T>::operator()...;
+  };
+  return OverloadImpl{std::forward<T>(ts)...};
+}
+#else
+namespace functional_internal {
+template <typename T>
+constexpr bool kDependentFalse = false;
+}
+
+template <typename Dependent = int, typename... T>
+auto Overload(T&&...) {
+  static_assert(functional_internal::kDependentFalse<Dependent>,
+                "Overload is only usable with C++17 or above.");
+}
+
+#endif
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_OVERLOAD_H_
diff --git a/absl/functional/overload_test.cc b/absl/functional/overload_test.cc
new file mode 100644
index 00000000..739c4c4c
--- /dev/null
+++ b/absl/functional/overload_test.cc
@@ -0,0 +1,130 @@
+// 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/functional/overload.h"
+
+#include <cstdint>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/variant.h"
+
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
+
+#include "gtest/gtest.h"
+
+namespace {
+
+TEST(OverloadTest, DispatchConsidersType) {
+  auto overloaded = absl::Overload(
+      [](int v) -> std::string { return absl::StrCat("int ", v); },        //
+      [](double v) -> std::string { return absl::StrCat("double ", v); },  //
+      [](const char* v) -> std::string {                                   //
+        return absl::StrCat("const char* ", v);                            //
+      },                                                                   //
+      [](auto v) -> std::string { return absl::StrCat("auto ", v); }       //
+  );
+  EXPECT_EQ("int 1", overloaded(1));
+  EXPECT_EQ("double 2.5", overloaded(2.5));
+  EXPECT_EQ("const char* hello", overloaded("hello"));
+  EXPECT_EQ("auto 1.5", overloaded(1.5f));
+}
+
+TEST(OverloadTest, DispatchConsidersNumberOfArguments) {
+  auto overloaded = absl::Overload(                 //
+      [](int a) { return a + 1; },                  //
+      [](int a, int b) { return a * b; },           //
+      []() -> absl::string_view { return "none"; }  //
+  );
+  EXPECT_EQ(3, overloaded(2));
+  EXPECT_EQ(21, overloaded(3, 7));
+  EXPECT_EQ("none", overloaded());
+}
+
+TEST(OverloadTest, SupportsConstantEvaluation) {
+  auto overloaded = absl::Overload(                 //
+      [](int a) { return a + 1; },                  //
+      [](int a, int b) { return a * b; },           //
+      []() -> absl::string_view { return "none"; }  //
+  );
+  static_assert(overloaded() == "none");
+  static_assert(overloaded(2) == 3);
+  static_assert(overloaded(3, 7) == 21);
+}
+
+TEST(OverloadTest, PropogatesDefaults) {
+  auto overloaded = absl::Overload(            //
+      [](int a, int b = 5) { return a * b; },  //
+      [](double c) { return c; }               //
+  );
+
+  EXPECT_EQ(21, overloaded(3, 7));
+  EXPECT_EQ(35, overloaded(7));
+  EXPECT_EQ(2.5, overloaded(2.5));
+}
+
+TEST(OverloadTest, AmbiguousWithDefaultsNotInvocable) {
+  auto overloaded = absl::Overload(            //
+      [](int a, int b = 5) { return a * b; },  //
+      [](int c) { return c; }                  //
+  );
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+  static_assert(std::is_invocable_v<decltype(overloaded), int, int>);
+}
+
+TEST(OverloadTest, AmbiguousDuplicatesNotInvocable) {
+  auto overloaded = absl::Overload(  //
+      [](int a) { return a; },       //
+      [](int c) { return c; }        //
+  );
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+}
+
+TEST(OverloadTest, AmbiguousConversionNotInvocable) {
+  auto overloaded = absl::Overload(  //
+      [](uint16_t a) { return a; },  //
+      [](uint64_t c) { return c; }   //
+  );
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+}
+
+TEST(OverloadTest, DispatchConsidersSfinae) {
+  auto overloaded = absl::Overload(                    //
+      [](auto a) -> decltype(a + 1) { return a + 1; }  //
+  );
+  static_assert(std::is_invocable_v<decltype(overloaded), int>);
+  static_assert(!std::is_invocable_v<decltype(overloaded), std::string>);
+}
+
+TEST(OverloadTest, VariantVisitDispatchesCorrectly) {
+  absl::variant<int, double, std::string> v(1);
+  auto overloaded = absl::Overload(
+      [](int) -> absl::string_view { return "int"; },                   //
+      [](double) -> absl::string_view { return "double"; },             //
+      [](const std::string&) -> absl::string_view { return "string"; }  //
+  );
+  EXPECT_EQ("int", absl::visit(overloaded, v));
+  v = 1.1;
+  EXPECT_EQ("double", absl::visit(overloaded, v));
+  v = "hello";
+  EXPECT_EQ("string", absl::visit(overloaded, v));
+}
+
+}  // namespace
+
+#endif