[libc++][stringbuf] Test and document LWG2995. (#100879)
As mentioned in the LWG issue libc++ has already implemented the
optimization. This adds tests and documents the implementation defined
behaviour.
Drive-by fixes an initialization.
diff --git a/libcxx/docs/ImplementationDefinedBehavior.rst b/libcxx/docs/ImplementationDefinedBehavior.rst
index f0ef733..1f95de7 100644
--- a/libcxx/docs/ImplementationDefinedBehavior.rst
+++ b/libcxx/docs/ImplementationDefinedBehavior.rst
@@ -62,6 +62,14 @@
- ``std::hermite(unsigned n, T x)`` for ``n >= 128``
+`[stringbuf.cons] <http://eel.is/c++draft/stringbuf.cons>`_ Whether sequence pointers are initialized to null pointers
+----------------------------------------------------------------------------------------------------------------------
+
+Libc++ does not initialize the pointers to null pointers. It resizes the buffer
+to its capacity and uses that size. This means the SSO buffer of
+``std::string`` is used as initial output buffer.
+
+
Listed in the index of implementation-defined behavior
======================================================
