Add InPlaceFunction to mediautils
This utility class is an almost drop-in replacement for std::function
which never allocates, and always stores the functional object in an
in-line buffer of parametrizable size.
Test: atest inplace_function_tests
Bug: 238654698
Change-Id: I48a7c16fad7673ccf03f8ae83036f2c96d76e994
diff --git a/media/utils/include/mediautils/InPlaceFunction.h b/media/utils/include/mediautils/InPlaceFunction.h
new file mode 100644
index 0000000..8fa23e8
--- /dev/null
+++ b/media/utils/include/mediautils/InPlaceFunction.h
@@ -0,0 +1,267 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include <cstdlib>
+#include <functional>
+#include <memory>
+#include <type_traits>
+
+namespace android::mediautils {
+
+namespace detail {
+// Vtable interface for erased types
+template <typename Ret, typename... Args>
+struct ICallableTable {
+ // Destroy the erased type
+ void (*destroy)(void* storage) = nullptr;
+ // Call the erased object
+ Ret (*invoke)(void* storage, Args...) = nullptr;
+ // **Note** the next two functions only copy object data, not the vptr
+ // Copy the erased object to a new InPlaceFunction buffer
+ void (*copy_to)(const void* storage, void* other) = nullptr;
+ // Move the erased object to a new InPlaceFunction buffer
+ void (*move_to)(void* storage, void* other) = nullptr;
+};
+} // namespace detail
+
+// This class is an *almost* drop-in replacement for std::function which is guaranteed to never
+// allocate, and always holds the type erased functional object in an in-line small buffer of
+// templated size. If the object is too large to hold, the type will fail to instantiate.
+//
+// Two notable differences are:
+// - operator() is not const (unlike std::function where the call operator is
+// const even if the erased type is not const callable). This retains const
+// correctness by default. A workaround is keeping InPlaceFunction mutable.
+// - Moving from an InPlaceFunction leaves the object in a valid state (operator
+// bool remains true), similar to std::optional/std::variant.
+// Calls to the object are still defined (and are equivalent
+// to calling the underlying type after it has been moved from). To opt-out
+// (and/or ensure safety), clearing the object is recommended:
+// func1 = std::move(func2); // func2 still valid (and moved-from) after this line
+// func2 = nullptr; // calling func2 will now abort
+template <typename, size_t BufferSize = 32>
+class InPlaceFunction;
+// We partially specialize to match types which are spelled like functions
+template <typename Ret, typename... Args, size_t BufferSize>
+class InPlaceFunction<Ret(Args...), BufferSize> {
+ public:
+ // Storage Type Details
+ static constexpr size_t Size = BufferSize;
+ static constexpr size_t Alignment = alignof(std::max_align_t);
+ using Buffer_t = std::aligned_storage_t<Size, Alignment>;
+ template <typename T, size_t Other>
+ friend class InPlaceFunction;
+
+ private:
+ // Callable which is used for empty InPlaceFunction objects (to match the
+ // std::function interface).
+ struct BadCallable {
+ [[noreturn]] Ret operator()(Args...) { std::abort(); }
+ };
+ static_assert(std::is_trivially_destructible_v<BadCallable>);
+
+ // Implementation of vtable interface for erased types.
+ // Contains only static vtable instantiated once for each erased type and
+ // static helpers.
+ template <typename T>
+ struct TableImpl {
+ // T should be a decayed type
+ static_assert(std::is_same_v<T, std::decay_t<T>>);
+
+ // Helper functions to get an unerased reference to the type held in the
+ // buffer. std::launder is required to avoid strict aliasing rules.
+ // The cast is always defined, as a precondition for these calls is that
+ // (exactly) a T was placement new constructed into the buffer.
+ constexpr static T& getRef(void* storage) {
+ return *std::launder(reinterpret_cast<T*>(storage));
+ }
+
+ constexpr static const T& getRef(const void* storage) {
+ return *std::launder(reinterpret_cast<const T*>(storage));
+ }
+
+ // Constexpr implies inline
+ constexpr static detail::ICallableTable<Ret, Args...> table = {
+ // Stateless lambdas are convertible to function ptrs
+ .destroy = [](void* storage) { getRef(storage).~T(); },
+ .invoke = [](void* storage, Args... args) -> Ret {
+ return std::invoke(getRef(storage), args...);
+ },
+ .copy_to = [](const void* storage,
+ void* other) { ::new (other) T(getRef(storage)); },
+ .move_to = [](void* storage,
+ void* other) { ::new (other) T(std::move(getRef(storage))); },
+ };
+ };
+
+ // Check size/align requirements for the T in Buffer_t. We use a templated
+ // struct to enable std::conjunction (see below).
+ template <typename T>
+ struct WillFit : std::integral_constant<bool, sizeof(T) <= Size && alignof(T) <= Alignment> {};
+
+ // Check size/align requirements for a function to function conversion
+ template <typename T>
+ struct ConversionWillFit
+ : std::integral_constant<bool, (T::Size < Size) && (T::Alignment <= Alignment)> {};
+ template <typename T>
+ struct IsInPlaceFunction : std::false_type {};
+
+ template <size_t BufferSize_>
+ struct IsInPlaceFunction<InPlaceFunction<Ret(Args...), BufferSize_>> : std::true_type {};
+
+ // Pred is true iff T is a valid type to construct an InPlaceFunction with
+ // We use std::conjunction for readability and short-circuit behavior
+ // (checks are ordered).
+ // The actual target type is the decay of T.
+ template <typename T>
+ static constexpr bool Pred = std::conjunction_v<
+ std::negation<IsInPlaceFunction<std::decay_t<T>>>, // T is not also an InPlaceFunction
+ // of the same signature.
+ std::is_invocable_r<Ret, std::decay_t<T>, Args...>, // correct signature callable
+ WillFit<std::decay_t<T>> // The target type fits in local storage
+ >;
+
+ template <typename T>
+ static constexpr bool ConvertibleFunc =
+ std::conjunction_v<IsInPlaceFunction<std::decay_t<T>>, // implies correctly invokable
+ ConversionWillFit<std::decay_t<T>>>;
+
+ // Members below
+ // This must come first for alignment
+ Buffer_t storage_;
+ const detail::ICallableTable<Ret, Args...>* vptr_;
+
+ constexpr void copy_to(InPlaceFunction& other) const {
+ vptr_->copy_to(std::addressof(storage_), std::addressof(other.storage_));
+ other.vptr_ = vptr_;
+ }
+
+ constexpr void move_to(InPlaceFunction& other) {
+ vptr_->move_to(std::addressof(storage_), std::addressof(other.storage_));
+ other.vptr_ = vptr_;
+ }
+
+ constexpr void destroy() { vptr_->destroy(std::addressof(storage_)); }
+
+ template <typename T, typename Target = std::decay_t<T>>
+ constexpr void genericInit(T&& t) {
+ vptr_ = &TableImpl<Target>::table;
+ ::new (std::addressof(storage_)) Target(std::forward<T>(t));
+ }
+
+ template <typename T, typename Target = std::decay_t<T>>
+ constexpr void convertingInit(T&& smallerFunc) {
+ // Redundant, but just in-case
+ static_assert(Target::Size < Size && Target::Alignment <= Alignment);
+ if constexpr (std::is_lvalue_reference_v<T>) {
+ smallerFunc.vptr_->copy_to(std::addressof(smallerFunc.storage_),
+ std::addressof(storage_));
+ } else {
+ smallerFunc.vptr_->move_to(std::addressof(smallerFunc.storage_),
+ std::addressof(storage_));
+ }
+ vptr_ = smallerFunc.vptr_;
+ }
+
+ public:
+ // Public interface
+ template <typename T, std::enable_if_t<Pred<T>>* = nullptr>
+ constexpr InPlaceFunction(T&& t) {
+ genericInit(std::forward<T>(t));
+ }
+
+ // Conversion from smaller functions.
+ template <typename T, std::enable_if_t<ConvertibleFunc<T>>* = nullptr>
+ constexpr InPlaceFunction(T&& t) {
+ convertingInit(std::forward<T>(t));
+ }
+
+ constexpr InPlaceFunction(const InPlaceFunction& other) { other.copy_to(*this); }
+
+ constexpr InPlaceFunction(InPlaceFunction&& other) { other.move_to(*this); }
+
+ // Making functions default constructible has pros and cons, we will align
+ // with the standard
+ constexpr InPlaceFunction() : InPlaceFunction(BadCallable{}) {}
+
+ constexpr InPlaceFunction(std::nullptr_t) : InPlaceFunction(BadCallable{}) {}
+#if __cplusplus >= 202002L
+ constexpr
+#endif
+ ~InPlaceFunction() {
+ destroy();
+ }
+
+ // The std::function call operator is marked const, but this violates const
+ // correctness. We deviate from the standard and do not mark the operator as
+ // const. Collections of InPlaceFunctions should probably be mutable.
+ constexpr Ret operator()(Args... args) {
+ return vptr_->invoke(std::addressof(storage_), args...);
+ }
+
+ constexpr InPlaceFunction& operator=(const InPlaceFunction& other) {
+ if (std::addressof(other) == this) return *this;
+ destroy();
+ other.copy_to(*this);
+ return *this;
+ }
+
+ constexpr InPlaceFunction& operator=(InPlaceFunction&& other) {
+ if (std::addressof(other) == this) return *this;
+ destroy();
+ other.move_to(*this);
+ return *this;
+ }
+
+ template <typename T, std::enable_if_t<Pred<T>>* = nullptr>
+ constexpr InPlaceFunction& operator=(T&& t) {
+ // We can't assign to ourselves, since T is a different type
+ destroy();
+ genericInit(std::forward<T>(t));
+ return *this;
+ }
+
+ // Explicitly defining this function saves a move/dtor
+ template <typename T, std::enable_if_t<ConvertibleFunc<T>>* = nullptr>
+ constexpr InPlaceFunction& operator=(T&& t) {
+ // We can't assign to ourselves, since T is different type
+ destroy();
+ convertingInit(std::forward<T>(t));
+ return *this;
+ }
+
+ constexpr InPlaceFunction& operator=(std::nullptr_t) { return operator=(BadCallable{}); }
+
+ // Moved from InPlaceFunctions are still considered valid (similar to
+ // std::optional). If using std::move on a function object explicitly, it is
+ // recommended that the object is reset using nullptr.
+ constexpr explicit operator bool() const { return vptr_ != &TableImpl<BadCallable>::table; }
+
+ constexpr void swap(InPlaceFunction& other) {
+ if (std::addressof(other) == this) return;
+ InPlaceFunction tmp{std::move(other)};
+ other.destroy();
+ move_to(other);
+ destroy();
+ tmp.move_to(*this);
+ }
+
+ friend constexpr void swap(InPlaceFunction& lhs, InPlaceFunction& rhs) { lhs.swap(rhs); }
+};
+
+} // namespace android::mediautils
diff --git a/media/utils/tests/Android.bp b/media/utils/tests/Android.bp
index f0e3c1e..58f86be 100644
--- a/media/utils/tests/Android.bp
+++ b/media/utils/tests/Android.bp
@@ -178,3 +178,13 @@
"extended_accumulator_tests.cpp",
],
}
+
+cc_test {
+ name: "inplace_function_tests",
+
+ defaults: ["libmediautils_tests_defaults"],
+
+ srcs: [
+ "inplace_function_tests.cpp"
+ ],
+}
diff --git a/media/utils/tests/inplace_function_tests.cpp b/media/utils/tests/inplace_function_tests.cpp
new file mode 100644
index 0000000..407c616
--- /dev/null
+++ b/media/utils/tests/inplace_function_tests.cpp
@@ -0,0 +1,346 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#define LOG_TAG "inplace_function_tests"
+
+#include <mediautils/InPlaceFunction.h>
+
+#include <type_traits>
+
+#include <gtest/gtest.h>
+#include <log/log.h>
+
+using namespace android;
+using namespace android::mediautils;
+
+struct BigCallable {
+ BigCallable(size_t* x, size_t val1, size_t val2) : ptr(x), a(val1), b(val2) {}
+ size_t* ptr;
+ size_t a;
+ size_t b;
+ size_t operator()(size_t input) const {
+ *ptr += a * 100 + b * 10 + input;
+ return 8;
+ }
+};
+
+TEST(InPlaceFunctionTests, Basic) {
+ size_t x = 5;
+ InPlaceFunction<size_t(size_t)> func;
+ {
+ BigCallable test{&x, 2, 3};
+ func = test;
+ }
+ EXPECT_EQ(func(2), 8ull);
+ EXPECT_EQ(x, 232ull + 5);
+}
+
+TEST(InPlaceFunctionTests, Invalid) {
+ InPlaceFunction<size_t(size_t)> func;
+ EXPECT_TRUE(!func);
+ InPlaceFunction<size_t(size_t)> func2{nullptr};
+ EXPECT_TRUE(!func2);
+ InPlaceFunction<size_t(size_t)> func3 = [](size_t x) { return x; };
+ EXPECT_TRUE(!(!func3));
+ func3 = nullptr;
+ EXPECT_TRUE(!func3);
+}
+
+TEST(InPlaceFunctionTests, MultiArg) {
+ InPlaceFunction<size_t(size_t, size_t, size_t)> func = [](size_t a, size_t b, size_t c) {
+ return a + b + c;
+ };
+ EXPECT_EQ(func(2, 3, 5), 2ull + 3 + 5);
+}
+struct Record {
+ Record(size_t m, size_t c, size_t d) : move_called(m), copy_called(c), dtor_called(d) {}
+ Record() {}
+ size_t move_called = 0;
+ size_t copy_called = 0;
+ size_t dtor_called = 0;
+ friend std::ostream& operator<<(std::ostream& os, const Record& record) {
+ return os << "Record, moves: " << record.move_called << ", copies: " << record.copy_called
+ << ", dtor: " << record.dtor_called << '\n';
+ }
+};
+
+bool operator==(const Record& lhs, const Record& rhs) {
+ return lhs.move_called == rhs.move_called && lhs.copy_called == rhs.copy_called &&
+ lhs.dtor_called == rhs.dtor_called;
+}
+
+struct Noisy {
+ Record& ref;
+ size_t state;
+ Noisy(Record& record, size_t val) : ref(record), state(val) {}
+ Noisy(const Noisy& other) : ref(other.ref), state(other.state) { ref.copy_called++; }
+
+ Noisy(Noisy&& other) : ref(other.ref), state(other.state) { ref.move_called++; }
+ ~Noisy() { ref.dtor_called++; }
+
+ size_t operator()() { return state; }
+};
+
+TEST(InPlaceFunctionTests, CtorForwarding) {
+ Record record;
+ Noisy noisy{record, 17};
+ InPlaceFunction<size_t()> func{noisy};
+ EXPECT_EQ(record, Record(0, 1, 0)); // move, copy, dtor
+ EXPECT_EQ(func(), 17ull);
+ Record record2;
+ Noisy noisy2{record2, 13};
+ InPlaceFunction<size_t()> func2{std::move(noisy2)};
+ EXPECT_EQ(record2, Record(1, 0, 0)); // move, copy, dtor
+ EXPECT_EQ(func2(), 13ull);
+}
+
+TEST(InPlaceFunctionTests, FunctionCtorForwarding) {
+ {
+ Record record;
+ Noisy noisy{record, 17};
+ InPlaceFunction<size_t()> func{noisy};
+ EXPECT_EQ(record, Record(0, 1, 0)); // move, copy, dtor
+ EXPECT_EQ(func(), 17ull);
+ InPlaceFunction<size_t()> func2{func};
+ EXPECT_EQ(record, Record(0, 2, 0)); // move, copy, dtor
+ EXPECT_EQ(func2(), 17ull);
+ }
+ Record record;
+ Noisy noisy{record, 13};
+ InPlaceFunction<size_t()> func{noisy};
+ EXPECT_EQ(record, Record(0, 1, 0)); // move, copy, dtor
+ EXPECT_EQ(func(), 13ull);
+ InPlaceFunction<size_t()> func2{std::move(func)};
+ EXPECT_EQ(record, Record(1, 1, 0)); // move, copy, dtor
+ EXPECT_EQ(func2(), 13ull);
+ // We expect moved from functions to still be valid
+ EXPECT_TRUE(!(!func));
+ EXPECT_EQ(static_cast<bool>(func), static_cast<bool>(func2));
+ EXPECT_EQ(func(), 13ull);
+}
+
+TEST(InPlaceFunctionTests, Dtor) {
+ Record record;
+ {
+ InPlaceFunction<size_t()> func;
+ {
+ Noisy noisy{record, 17};
+ func = noisy;
+ }
+ EXPECT_EQ(func(), 17ull);
+ EXPECT_EQ(record.dtor_called, 1ull);
+ }
+ EXPECT_EQ(record.dtor_called, 2ull);
+}
+
+TEST(InPlaceFunctionTests, Assignment) {
+ {
+ Record record;
+ Record record2;
+ Noisy noisy{record, 17};
+ Noisy noisy2{record2, 5};
+ InPlaceFunction<size_t()> func{noisy};
+ EXPECT_EQ(func(), 17ull);
+ EXPECT_EQ(record.dtor_called, 0ull);
+ func = noisy2;
+ EXPECT_EQ(record.dtor_called, 1ull);
+ EXPECT_EQ(record2, Record(0, 1, 0)); // move, copy, dtor
+ EXPECT_EQ(func(), 5ull);
+ }
+ {
+ Record record;
+ Record record2;
+ Noisy noisy{record, 17};
+ Noisy noisy2{record2, 5};
+ InPlaceFunction<size_t()> func{noisy};
+ EXPECT_EQ(func(), 17ull);
+ EXPECT_EQ(record.dtor_called, 0ull);
+ func = std::move(noisy2);
+ EXPECT_EQ(record.dtor_called, 1ull);
+ EXPECT_EQ(record2, Record(1, 0, 0)); // move, copy, dtor
+ EXPECT_EQ(func(), 5ull);
+ }
+
+ {
+ Record record;
+ Record record2;
+ Noisy noisy{record, 17};
+ Noisy noisy2{record2, 13};
+ {
+ InPlaceFunction<size_t()> func{noisy};
+ EXPECT_EQ(func(), 17ull);
+ InPlaceFunction<size_t()> func2{noisy2};
+ EXPECT_EQ(record2, Record(0, 1, 0)); // move, copy, dtor
+ EXPECT_EQ(record.dtor_called, 0ull);
+ func = func2;
+ EXPECT_EQ(record.dtor_called, 1ull);
+ EXPECT_EQ(func(), 13ull);
+ EXPECT_EQ(record2, Record(0, 2, 0)); // move, copy, dtor
+ EXPECT_TRUE(static_cast<bool>(func2));
+ EXPECT_EQ(func2(), 13ull);
+ }
+ EXPECT_EQ(record2, Record(0, 2, 2)); // move, copy, dtor
+ }
+
+ {
+ Record record;
+ Record record2;
+ Noisy noisy{record, 17};
+ Noisy noisy2{record2, 13};
+ {
+ InPlaceFunction<size_t()> func{noisy};
+ EXPECT_EQ(func(), 17ull);
+ InPlaceFunction<size_t()> func2{noisy2};
+ EXPECT_EQ(record.dtor_called, 0ull);
+ EXPECT_EQ(record2, Record(0, 1, 0)); // move, copy, dtor
+ func = std::move(func2);
+ EXPECT_EQ(record.dtor_called, 1ull);
+ EXPECT_EQ(func(), 13ull);
+ EXPECT_EQ(record2, Record(1, 1, 0)); // move, copy, dtor
+ // Moved from function is still valid
+ EXPECT_TRUE(static_cast<bool>(func2));
+ EXPECT_EQ(func2(), 13ull);
+ }
+ EXPECT_EQ(record2, Record(1, 1, 2)); // move, copy, dtor
+ }
+}
+
+TEST(InPlaceFunctionTests, Swap) {
+ Record record1;
+ Record record2;
+ InPlaceFunction<size_t()> func1 = Noisy{record1, 5};
+ InPlaceFunction<size_t()> func2 = Noisy{record2, 7};
+ EXPECT_EQ(record1, Record(1, 0, 1)); // move, copy, dtor
+ EXPECT_EQ(record2, Record(1, 0, 1)); // move, copy, dtor
+ EXPECT_EQ(func1(), 5ull);
+ EXPECT_EQ(func2(), 7ull);
+ func1.swap(func2);
+ EXPECT_EQ(record1, Record(2, 0, 2)); // move, copy, dtor
+ // An additional move and destroy into the temporary object
+ EXPECT_EQ(record2, Record(3, 0, 3)); // move, copy, dtor
+ EXPECT_EQ(func1(), 7ull);
+ EXPECT_EQ(func2(), 5ull);
+}
+
+TEST(InPlaceFunctionTests, Conversion) {
+ Record record;
+ Noisy noisy{record, 15};
+ {
+ InPlaceFunction<size_t(), 16> func2 = noisy;
+ EXPECT_EQ(record, Record(0, 1, 0)); // move, copy, dtor
+ {
+ InPlaceFunction<size_t(), 32> func{func2};
+ EXPECT_EQ(record, Record(0, 2, 0)); // move, copy, dtor
+ EXPECT_EQ(func2(), func());
+ }
+ EXPECT_EQ(record, Record(0, 2, 1)); // move, copy, dtor
+ }
+ EXPECT_EQ(record, Record(0, 2, 2)); // move, copy, dtor
+}
+
+TEST(InPlaceFunctionTests, ConversionMove) {
+ Record record;
+ Noisy noisy{record, 15};
+ {
+ InPlaceFunction<size_t(), 16> func2 = noisy;
+ EXPECT_EQ(record, Record(0, 1, 0)); // move, copy, dtor
+ {
+ InPlaceFunction<size_t(), 32> func{std::move(func2)};
+ EXPECT_EQ(record, Record(1, 1, 0)); // move, copy, dtor
+ EXPECT_EQ(func2(), func());
+ }
+ EXPECT_EQ(record, Record(1, 1, 1)); // move, copy, dtor
+ }
+ EXPECT_EQ(record, Record(1, 1, 2)); // move, copy, dtor
+}
+
+TEST(InPlaceFunctionTests, ConversionAssign) {
+ Record record;
+ Noisy noisy{record, 15};
+ {
+ InPlaceFunction<size_t(), 32> func;
+ {
+ InPlaceFunction<size_t(), 16> func2 = noisy;
+ EXPECT_EQ(record, Record(0, 1, 0)); // move, copy, dtor
+ func = func2;
+ EXPECT_EQ(record, Record(0, 2, 0)); // move, copy, dtor
+ EXPECT_EQ(func2(), func());
+ }
+ EXPECT_EQ(record, Record(0, 2, 1)); // move, copy, dtor
+ }
+ EXPECT_EQ(record, Record(0, 2, 2)); // move, copy, dtor
+}
+
+TEST(InPlaceFunctionTests, ConversionAssignMove) {
+ Record record;
+ Noisy noisy{record, 15};
+ {
+ InPlaceFunction<size_t(), 32> func;
+ {
+ InPlaceFunction<size_t(), 16> func2 = noisy;
+ EXPECT_EQ(record, Record(0, 1, 0)); // move, copy, dtor
+ func = std::move(func2);
+ EXPECT_EQ(record, Record(1, 1, 0)); // move, copy, dtor
+ EXPECT_EQ(func2(), func());
+ }
+ EXPECT_EQ(record, Record(1, 1, 1)); // move, copy, dtor
+ }
+ EXPECT_EQ(record, Record(1, 1, 2)); // move, copy, dtor
+}
+
+template <class T, class U, class = void>
+struct can_assign : std::false_type {};
+
+template <class T, class U>
+struct can_assign<T, U, typename std::void_t<decltype(T().operator=(U()))>> : std::true_type {};
+
+template <class From, class To, bool Expected>
+static constexpr bool Convertible =
+ (can_assign<To, From>::value ==
+ std::is_constructible_v<To, From>)&&(std::is_constructible_v<To, From> == Expected);
+
+struct TooBig {
+ std::array<uint64_t, 5> big = {1, 2, 3, 4, 5};
+ size_t operator()() { return static_cast<size_t>(big[0] + big[1] + big[2] + big[3] + big[4]); }
+};
+static_assert(sizeof(TooBig) == 40);
+struct NotCallable {};
+struct WrongArg {
+ void operator()(NotCallable) {}
+};
+struct WrongRet {
+ NotCallable operator()(size_t) { return NotCallable{}; }
+};
+
+static_assert(Convertible<InPlaceFunction<size_t(), 32>, InPlaceFunction<size_t(), 32>, true>);
+static_assert(
+ Convertible<InPlaceFunction<size_t(size_t), 32>, InPlaceFunction<size_t(), 32>, false>);
+static_assert(Convertible<InPlaceFunction<void(), 32>, InPlaceFunction<size_t(), 32>, false>);
+static_assert(Convertible<TooBig, InPlaceFunction<size_t(), 32>, false>);
+static_assert(Convertible<TooBig, InPlaceFunction<size_t(), 40>, true>);
+static_assert(Convertible<NotCallable, InPlaceFunction<size_t(), 40>, false>);
+static_assert(Convertible<WrongArg, InPlaceFunction<void(size_t), 40>, false>);
+static_assert(Convertible<WrongRet, InPlaceFunction<size_t(size_t), 40>, false>);
+// Void returning functions are modelled by any return type
+static_assert(Convertible<WrongRet, InPlaceFunction<void(size_t), 40>, true>);
+
+// Check constructibility/assignability from smaller function types
+static_assert(Convertible<InPlaceFunction<size_t(), 32>, InPlaceFunction<size_t(), 24>, false>);
+static_assert(Convertible<InPlaceFunction<size_t(), 32>, InPlaceFunction<size_t(), 40>, true>);
+static_assert(
+ Convertible<InPlaceFunction<size_t(), 32>, InPlaceFunction<size_t(size_t), 40>, false>);
+static_assert(
+ Convertible<InPlaceFunction<size_t(), 32>, InPlaceFunction<NotCallable(), 40>, false>);