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).

1 files changed, 6 insertions, 1 deletions

diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index 5d81bfc..75a2d88 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();