[libc++] Fixes use-after move diagnostic.

[Mordante]

Summary

The diagnostic is issued by clang-tidy 17.

This just suppressed the diagnostic. The move operations are non-standard extensions and the class itself is deprecated.

[Mordante]

Rebased and a different approach.

[Mordante]

Rebased to fix the CI.

[ldionne]

libcxx/include/strstream

261

What is the non-standard extension? I don't understand why this code is valid and this isn't a bug? Does istream(_VSTD::move(__rhs)) above not move the __sb_ member? If not, would it be possible to instead do something like

istream(std::move(static_cast<istream&>(__rhs)))

to make it clear that we're only moving __rhs as a base class object (if that's what's going on)?

[Mordante]

libcxx/include/strstream

261

The non-Standard part are the move operations. I've updated the description of the patch.

I can make that change, it looks closer to my original edit istream(static_cast<istream&&>(__rhs)), but this is less subtle :-)

[ldionne]

libcxx/include/strstream

258–259

Here and below.

261

Yeah I think that makes it a bit more obvious. We might not even need to add a NOLINT annotation then?

[philnik]

I still don't understand what extension is supposed to be used. Not a single compiler complains about casting an rvalue to a base: https://godbolt.org/z/bv4Moh5on

libcxx/include/strstream

261

TBH the std::move makes more sense to me than the static_cast, which IMO would warrant a comment.

[Mordante]

Addresses review comments.

[Mordante]

libcxx/include/strstream

261

I've switched to Louis' suggestion only kept the _VSTD to keep the patch minimal.

[ldionne]

libcxx/include/strstream

260

Don't you mean istream& here instead? You want to cast to the base class?

309

Same here.

369

This one looks right.

[Mordante]

Addresses review comments.

[Mordante]

Thanks for catching me copy-pasting from the wrong place.

libcxx/include/strstream

public:
     _LIBCPP_INLINE_VISIBILITY
     istrstream(const char* __s, streamsize __n)
         : istream(&__sb_), __sb_(__s, __n) {}
     _LIBCPP_INLINE_VISIBILITY
     istrstream(char* __s, streamsize __n)
         : istream(&__sb_), __sb_(__s, __n) {}
 
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
-    istrstream(istrstream&& __rhs)
-        : istream(_VSTD::move(__rhs)),
+    istrstream(istrstream&& __rhs)  // extension
+        : istream(_VSTD::move(static_cast<istream&>(__rhs))),
           __sb_(_VSTD::move(__rhs.__sb_))
     {
         istream::set_rdbuf(&__sb_);
     }
 
     _LIBCPP_INLINE_VISIBILITY
     istrstream& operator=(istrstream&& __rhs)
     {
         __sb_ = _VSTD::move(__rhs.__sb_);
public:
     _LIBCPP_INLINE_VISIBILITY
     ostrstream(char* __s, int __n, ios_base::openmode __mode = ios_base::out)
         : ostream(&__sb_),
           __sb_(__s, __n, __s + (__mode & ios::app ? _VSTD::strlen(__s) : 0))
         {}
 
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
-    ostrstream(ostrstream&& __rhs)
-        : ostream(_VSTD::move(__rhs)),
+    ostrstream(ostrstream&& __rhs)  // extension
+        : ostream(_VSTD::move(static_cast<ostream&>(__rhs))),
           __sb_(_VSTD::move(__rhs.__sb_))
     {
         ostream::set_rdbuf(&__sb_);
     }
 
     _LIBCPP_INLINE_VISIBILITY
     ostrstream& operator=(ostrstream&& __rhs)
     {
public:
     _LIBCPP_INLINE_VISIBILITY
     strstream(char* __s, int __n, ios_base::openmode __mode = ios_base::in | ios_base::out)
         : iostream(&__sb_),
           __sb_(__s, __n, __s + (__mode & ios::app ? _VSTD::strlen(__s) : 0))
         {}
 
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
-    strstream(strstream&& __rhs)
-        : iostream(_VSTD::move(__rhs)),
+    strstream(strstream&& __rhs)  // extension
+        : iostream(_VSTD::move(static_cast<iostream&>(__rhs))),
           __sb_(_VSTD::move(__rhs.__sb_))
     {
         iostream::set_rdbuf(&__sb_);
     }
 
     _LIBCPP_INLINE_VISIBILITY
     strstream& operator=(strstream&& __rhs)
     {