4f4690530e
Currently std::expected can have some padding bytes in its tail due to
[[no_unique_address]]. Those padding bytes can be used by other objects.
For example, in the current implementation:
sizeof(std::expected<std::optional<int>, bool>) ==
sizeof(std::expected<std::expected<std::optional<int>, bool>, bool>)
As a result, the data layout of an
std::expected<std::expected<std::optional<int>, bool>, bool>
can look like this:
+-- optional "has value" flag
| +--padding
/---int---\ | |
00 00 00 00 01 00 00 00
| |
| +- "outer" expected "has value" flag
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+- expected "has value" flag
This is problematic because `emplace()`ing the "inner" expected can not
only overwrite the "inner" expected "has value" flag (issue #68552) but
also the tail padding where other objects might live.
This patch fixes the problem by ensuring that std::expected has no tail
padding, which is achieved by conditional usage of [[no_unique_address]]
based on the tail padding that this would create.
This is an ABI breaking change because the following property changes:
sizeof(std::expected<std::optional<int>, bool>) <
sizeof(std::expected<std::expected<std::optional<int>, bool>, bool>)
Before the change, this relation didn't hold. After the change, the relation
does hold, which means that the size of std::expected in these cases increases
after this patch. The data layout will change in the following cases where
tail padding can be reused by other objects:
class foo : std::expected<std::optional<int>, bool> {
bool b;
};
or using [[no_unique_address]]:
struct foo {
[[no_unique_address]] std::expected<std::optional<int>, bool> e;
bool b;
};
The vendor communication is handled in #70820.
Fixes: #70494
Co-authored-by: philnik777 <nikolasklauser@berlin.de>
Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
132 lines
3.7 KiB
C++
132 lines
3.7 KiB
C++
//===----------------------------------------------------------------------===//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
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// template<class G>
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// constexpr expected& operator=(const unexpected<G>& e);
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//
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// Let GF be const G&
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//
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// Constraints: is_constructible_v<E, GF> is true and is_assignable_v<E&, GF> is true.
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//
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// Effects:
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// - If has_value() is true, equivalent to:
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// construct_at(addressof(unex), std::forward<GF>(e.error()));
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// has_val = false;
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// - Otherwise, equivalent to: unex = std::forward<GF>(e.error());
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//
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// Returns: *this.
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#include <cassert>
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#include <concepts>
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#include <expected>
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#include <type_traits>
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#include <utility>
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#include "../../types.h"
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#include "test_macros.h"
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struct NotCopyConstructible {
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NotCopyConstructible(const NotCopyConstructible&) = delete;
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NotCopyConstructible& operator=(const NotCopyConstructible&) = default;
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};
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struct NotCopyAssignable {
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NotCopyAssignable(const NotCopyAssignable&) = default;
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NotCopyAssignable& operator=(const NotCopyAssignable&) = delete;
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};
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struct MoveMayThrow {
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MoveMayThrow(MoveMayThrow const&) = default;
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MoveMayThrow& operator=(const MoveMayThrow&) = default;
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MoveMayThrow(MoveMayThrow&&) noexcept(false) {}
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MoveMayThrow& operator=(MoveMayThrow&&) noexcept(false) { return *this; }
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};
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// Test constraints
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static_assert(std::is_assignable_v<std::expected<void, int>&, const std::unexpected<int>&>);
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// !is_constructible_v<E, GF>
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static_assert(
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!std::is_assignable_v<std::expected<void, NotCopyConstructible>&, const std::unexpected<NotCopyConstructible>&>);
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// !is_assignable_v<E&, GF>
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static_assert(
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!std::is_assignable_v<std::expected<void, NotCopyAssignable>&, const std::unexpected<NotCopyAssignable>&>);
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constexpr bool test() {
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// - If has_value() is true, equivalent to:
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// construct_at(addressof(unex), std::forward<GF>(e.error()));
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// has_val = false;
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{
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Traced::state state{};
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std::expected<void, Traced> e;
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std::unexpected<Traced> un(std::in_place, state, 5);
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decltype(auto) x = (e = un);
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static_assert(std::same_as<decltype(x), std::expected<void, Traced>&>);
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assert(&x == &e);
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assert(!e.has_value());
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assert(e.error().data_ == 5);
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assert(state.copyCtorCalled);
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}
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// - Otherwise, equivalent to: unex = std::forward<GF>(e.error());
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{
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Traced::state state1{};
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Traced::state state2{};
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std::expected<void, Traced> e(std::unexpect, state1, 5);
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std::unexpected<Traced> un(std::in_place, state2, 10);
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decltype(auto) x = (e = un);
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static_assert(std::same_as<decltype(x), std::expected<void, Traced>&>);
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assert(&x == &e);
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assert(!e.has_value());
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assert(e.error().data_ == 10);
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assert(state1.copyAssignCalled);
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}
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// CheckForInvalidWrites
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{
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{
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CheckForInvalidWrites<true, true> e;
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std::unexpected<int> un(std::in_place, 42);
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e = un;
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assert(e.check());
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}
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{
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CheckForInvalidWrites<false, true> e;
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std::unexpected<bool> un(std::in_place, true);
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e = un;
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assert(e.check());
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}
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}
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return true;
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}
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void testException() {
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#ifndef TEST_HAS_NO_EXCEPTIONS
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std::expected<void, ThrowOnCopyConstruct> e1(std::in_place);
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std::unexpected<ThrowOnCopyConstruct> un(std::in_place);
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try {
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e1 = un;
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assert(false);
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} catch (Except) {
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assert(e1.has_value());
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}
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#endif // TEST_HAS_NO_EXCEPTIONS
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}
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int main(int, char**) {
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test();
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static_assert(test());
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testException();
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return 0;
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}
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