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.

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 cfbc78a..99fdbee 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 a336b55..f716030 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;
   });
 }