Internal change

PiperOrigin-RevId: 523797476

mediapipe/framework/deps/BUILD

@@ -305,6 +305,14 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "requires",
+    hdrs = ["requires.h"],
+
+    # Use this library through "mediapipe/framework/port:requires".
+    visibility = ["//mediapipe/framework/port:__pkg__"],
+)
+
 cc_library(
     name = "ret_check",
     srcs = ["ret_check.cc"],

mediapipe/framework/deps/requires.h

@@ -0,0 +1,61 @@
+// Copyright 2023 The MediaPipe 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
+//
+//      http://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 MEDIAPIPE_DEPS_REQUIRES_H_
+#define MEDIAPIPE_DEPS_REQUIRES_H_
+
+#include <type_traits>
+
+namespace mediapipe {
+
+// C++17 port of the C++20 `requires` expressions.
+// It allows easy inline test of properties of types in template code.
+// https://en.cppreference.com/w/cpp/language/constraints#Requires_expressions
+//
+// Example usage:
+//
+// if constexpr (Requires<T>([](auto&& x) -> decltype(x.foo()) {})) {
+//   // T has foo()
+//   return t.foo();
+// } else if constexpr (Requires<T>([](auto&& x) -> decltype(Bar(x)) {})) {
+//   // Can call Bar with T
+//   return Bar(t);
+// } else if constexpr (Requires<T, U>(
+//     // Can test expression with multiple inputs
+//     [](auto&& x, auto&& y) -> decltype(x + y) {})) {
+//   return t + t2;
+// }
+//
+// The `Requires` function takes a list of types and a generic lambda where all
+// arguments are of type `auto&&`. The lambda is never actually invoked and the
+// body must be empty.
+// When used this way, `Requires` returns whether the expression inside
+// `decltype` is well-formed, when the lambda parameters have the types that
+// are specified by the corresponding template arguments.
+//
+// NOTE: C++17 does not allow lambdas in template parameters, which means that
+// code like the following is _not_ valid in C++17:
+//
+//  template <typename T,
+//            typename = std::enable_if_t<Requires<T>(
+//              [] (auto&& v) -> decltype(<expr>) {})>>
+//
+template <typename... T, typename F>
+constexpr bool Requires(F) {
+  return std::is_invocable_v<F, T...>;
+}
+
+}  // namespace mediapipe
+
+#endif  // MEDIAPIPE_DEPS_REQUIRES_H_

mediapipe/framework/port/BUILD

@@ -445,3 +445,9 @@ cc_library(
     hdrs = ["vector.h"],
     deps = ["//mediapipe/framework/deps:vector"],
 )
+
+cc_library(
+    name = "requires",
+    hdrs = ["requires.h"],
+    deps = ["//mediapipe/framework/deps:requires"],
+)

mediapipe/framework/port/requires.h

@@ -0,0 +1,20 @@
+// Copyright 2023 The MediaPipe 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
+//
+//      http://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 MEDIAPIPE_FRAMEWORK_PORT_REQUIRES_H_
+#define MEDIAPIPE_FRAMEWORK_PORT_REQUIRES_H_
+
+#include "mediapipe/framework/deps/requires.h"
+
+#endif  // MEDIAPIPE_FRAMEWORK_PORT_REQUIRES_H_