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

1 files changed, 79 insertions, 0 deletions

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