[gcc r15-2650] libstdc++: Remove noexcept from non-const std::basic_string::data() [PR99942]

[Jonathan Wakely]

https://gcc.gnu.org/g:6586b015f1211ccd6e3e89b44dcb2116347edf89

commit r15-2650-g6586b015f1211ccd6e3e89b44dcb2116347edf89
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 27 11:07:17 2024 +0000

    libstdc++: Remove noexcept from non-const std::basic_string::data() [PR99942]
    
    The C++17 non-const overload of data() allows modifying the string
    contents directly, so for the COW string we must do a copy-on-write to
    unshare it. That means allocating, which can throw, so it shouldn't be
    noexcept.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/99942
            * include/bits/cow_string.h (data()): Change to noexcept(false).

Diff:
---
 libstdc++-v3/include/bits/cow_string.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index 5d81bfc1230e..75a2d887ad63 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -2267,9 +2267,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  This is a pointer to the character sequence held by the string.
        *  Modifying the characters in the sequence is allowed.
+       *
+       *  The standard requires this function to be `noexcept` but for the
+       *  Copy-On-Write string implementation it can throw.  This function
+       *  allows modifying the string contents directly, which means we
+       *  must copy-on-write to unshare it, which requires allocating memory.
       */
       _CharT*
-      data() noexcept
+      data() noexcept(false)
       {
 	_M_leak();
 	return _M_data();