[gcc r13-6808] libstdc++: Use rvalues in std::string::resize_and_overwrite (LWG 3645)

[gcc r13-6808] libstdc++: Use rvalues in std::string::resize_and_overwrite (LWG 3645)

[Jonathan Wakely]

https://gcc.gnu.org/g:ba4f5530c475eadd2a7edb46b6c1c9d5f9267501

commit r13-6808-gba4f5530c475eadd2a7edb46b6c1c9d5f9267501
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 22 11:54:31 2023 +0000

    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.

Diff:
---
 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 cfbc78a3108..99fdbeee5ad 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 a336b55f4a1..f716030dad7 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;
   });
 }