From ba4f5530c475eadd2a7edb46b6c1c9d5f9267501 Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Wed, 22 Mar 2023 11:54:31 +0000
Subject: [PATCH] libstdc++: Use rvalues in std::string::resize_and_overwrite
(LWG 3645)
Previously the C++23 draft required that the callback arguments were
lvalues, which was overvable by the callback. LWG 3645 removes that
overspecification, so we can pass rvalues and the user can't modify
our local variables. I've used auto(p) to produce rvalues, which is only
supported since Clang 15, but I think that's OK for a C++23 feature.
While making this change I noticed that we weren't correctly enforcing
the requirement that the callback returns an integer-like type. Add
better assertions for the type and value.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.tcc (basic_string::resize_and_overwrite):
Pass rvalues to the callback, as now allowed by LWG 3645.
Enforce preconditions on the return value.
* testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc:
Adjust.
---
libstdc++-v3/include/bits/basic_string.tcc | 9 ++++++---
.../basic_string/capacity/char/resize_and_overwrite.cc | 8 +++++---
2 files changed, 11 insertions(+), 6 deletions(-)
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index cfbc78a31081..99fdbeee5ad9 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -592,9 +592,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
size_type _M_r;
};
_Terminator __term{this};
- const size_type __n2 [[maybe_unused]] = __n;
- __term._M_r = std::move(__op)(__p, __n);
- _GLIBCXX_DEBUG_ASSERT(__term._M_r >= 0 && __term._M_r <= __n2);
+ auto __r = std::move(__op)(auto(__p), auto(__n));
+ static_assert(ranges::__detail::__is_integer_like<decltype(__r)>);
+ _GLIBCXX_DEBUG_ASSERT(__r >= 0 && __r <= __n);
+ __term._M_r = size_type(__r);
+ if (__term._M_r > __n)
+ __builtin_unreachable();
}
#endif // C++23
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc b/libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc
index a336b55f4a11..f716030dad7f 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc
@@ -84,9 +84,11 @@ test03()
VERIFY( s == std::string(42, 'a') );
VERIFY( s[42] == '\0' );
- s.resize_and_overwrite(0, [](auto&& p, auto&& n) {
- static_assert( std::is_same_v<decltype(p), char*&> );
- static_assert( std::is_same_v<decltype(n), std::string::size_type&> );
+ s.resize_and_overwrite(0, [](auto p, auto n) {
+ // N.B. these requirements were relaxed by LWG 3645:
+ // resize_and_overwrite is overspecified to call its callback with lvalues
+ static_assert( std::is_same_v<decltype(p), char*> );
+ static_assert( std::is_same_v<decltype(n), std::string::size_type> );
return 0;
});
}
--
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