Introduce IfConstexpr(Else) utilities for use internally in abseil library implementation.

These functions are meant to allow for avoiding complex SFINAE pre-C++17.

PiperOrigin-RevId: 527968428
Change-Id: I6524206c9dc50663d7c38abf6d0e8080ca93fc30

4 files changed, 196 insertions, 0 deletions

diff --git a/absl/utility/BUILD.bazel b/absl/utility/BUILD.bazel
index ca4bc0a6..061f4c5b 100644
--- a/absl/utility/BUILD.bazel
+++ b/absl/utility/BUILD.bazel
@@ -52,3 +52,26 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+cc_library(
+    name = "if_constexpr",
+    hdrs = [
+        "internal/if_constexpr.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+    ],
+)
+
+cc_test(
+    name = "if_constexpr_test",
+    srcs = ["internal/if_constexpr_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":if_constexpr",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
diff --git a/absl/utility/CMakeLists.txt b/absl/utility/CMakeLists.txt
index 865b758f..27ee0de5 100644
--- a/absl/utility/CMakeLists.txt
+++ b/absl/utility/CMakeLists.txt
@@ -42,3 +42,27 @@ absl_cc_test(
     absl::strings
     GTest::gmock_main
 )
+
+absl_cc_library(
+  NAME
+    if_constexpr
+  HDRS
+    "internal/if_constexpr.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    if_constexpr_test
+  SRCS
+    "internal/if_constexpr_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::if_constexpr
+    GTest::gmock_main
+)
diff --git a/absl/utility/internal/if_constexpr.h b/absl/utility/internal/if_constexpr.h
new file mode 100644
index 00000000..7a26311d
--- /dev/null
+++ b/absl/utility/internal/if_constexpr.h
@@ -0,0 +1,70 @@
+// 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.
+
+// The IfConstexpr and IfConstexprElse utilities in this file are meant to be
+// used to emulate `if constexpr` in pre-C++17 mode in library implementation.
+// The motivation is to allow for avoiding complex SFINAE.
+//
+// The functions passed in must depend on the type(s) of the object(s) that
+// require SFINAE. For example:
+// template<typename T>
+// int MaybeFoo(T& t) {
+//   if constexpr (HasFoo<T>::value) return t.foo();
+//   return 0;
+// }
+//
+// can be written in pre-C++17 as:
+//
+// template<typename T>
+// int MaybeFoo(T& t) {
+//   int i = 0;
+//   absl::utility_internal::IfConstexpr<HasFoo<T>::value>(
+//       [&](const auto& fooer) { i = fooer.foo(); }, t);
+//   return i;
+// }
+
+#ifndef ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
+#define ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
+
+#include <tuple>
+#include <utility>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace utility_internal {
+
+template <bool condition, typename TrueFunc, typename FalseFunc,
+          typename... Args>
+auto IfConstexprElse(TrueFunc&& true_func, FalseFunc&& false_func,
+                     Args&&... args) {
+  return std::get<condition>(std::forward_as_tuple(
+      std::forward<FalseFunc>(false_func), std::forward<TrueFunc>(true_func)))(
+      std::forward<Args>(args)...);
+}
+
+template <bool condition, typename Func, typename... Args>
+void IfConstexpr(Func&& func, Args&&... args) {
+  IfConstexprElse<condition>(std::forward<Func>(func), [](auto&&...){},
+                             std::forward<Args>(args)...);
+}
+
+}  // namespace utility_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
diff --git a/absl/utility/internal/if_constexpr_test.cc b/absl/utility/internal/if_constexpr_test.cc
new file mode 100644
index 00000000..d1ee7236
--- /dev/null
+++ b/absl/utility/internal/if_constexpr_test.cc
@@ -0,0 +1,79 @@
+// 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/utility/internal/if_constexpr.h"
+
+#include <utility>
+
+#include "gtest/gtest.h"
+
+namespace {
+
+struct Empty {};
+struct HasFoo {
+  int foo() const { return 1; }
+};
+
+TEST(IfConstexpr, Basic) {
+  int i = 0;
+  absl::utility_internal::IfConstexpr<false>(
+      [&](const auto& t) { i = t.foo(); }, Empty{});
+  EXPECT_EQ(i, 0);
+
+  absl::utility_internal::IfConstexpr<false>(
+      [&](const auto& t) { i = t.foo(); }, HasFoo{});
+  EXPECT_EQ(i, 0);
+
+  absl::utility_internal::IfConstexpr<true>(
+      [&](const auto& t) { i = t.foo(); }, HasFoo{});
+  EXPECT_EQ(i, 1);
+}
+
+TEST(IfConstexprElse, Basic) {
+  EXPECT_EQ(absl::utility_internal::IfConstexprElse<false>(
+      [&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; },
+      Empty{}), 2);
+
+  EXPECT_EQ(absl::utility_internal::IfConstexprElse<false>(
+      [&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; },
+      HasFoo{}), 2);
+
+  EXPECT_EQ(absl::utility_internal::IfConstexprElse<true>(
+      [&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; },
+      HasFoo{}), 1);
+}
+
+struct HasFooRValue {
+  int foo() && { return 1; }
+};
+struct RValueFunc {
+  void operator()(HasFooRValue&& t) && { *i = std::move(t).foo(); }
+
+  int* i = nullptr;
+};
+
+TEST(IfConstexpr, RValues) {
+  int i = 0;
+  RValueFunc func = {&i};
+  absl::utility_internal::IfConstexpr<false>(
+      std::move(func), HasFooRValue{});
+  EXPECT_EQ(i, 0);
+
+  func = RValueFunc{&i};
+  absl::utility_internal::IfConstexpr<true>(
+      std::move(func), HasFooRValue{});
+  EXPECT_EQ(i, 1);
+}
+
+}  // namespace