[libc++] Fix std::out_of_range thrown from basic_stringbuf::str() &&

Reviewed By: #libc, Mordante, philnik

Differential Revision: https://reviews.llvm.org/D156783

(cherry picked from commit f418cb1)

Author: pfusik

libcxx/include/sstream

@@ -399,8 +399,12 @@ public:
     _LIBCPP_HIDE_FROM_ABI_SSTREAM string_type str() const & { return str(__str_.get_allocator()); }
 
     _LIBCPP_HIDE_FROM_ABI_SSTREAM string_type str() && {
+        string_type __result;
         const basic_string_view<_CharT, _Traits> __view = view();
-        string_type __result(std::move(__str_), __view.data() - __str_.data(), __view.size());
+        if (!__view.empty()) {
+            auto __pos = __view.data() - __str_.data();
+            __result.assign(std::move(__str_), __pos, __view.size());
+        }
         __str_.clear();
         __init_buf_ptrs();
         return __result;

libcxx/test/std/input.output/string.streams/istringstream/istringstream.members/str.move.pass.cpp

@@ -31,6 +31,12 @@ static void test() {
     assert(s == STR("testing"));
     assert(ss.view().empty());
   }
+  {
+    std::basic_istringstream<CharT> ss;
+    std::basic_string<CharT> s = std::move(ss).str();
+    assert(s.empty());
+    assert(ss.view().empty());
+  }
 }
 
 int main(int, char**) {

libcxx/test/std/input.output/string.streams/ostringstream/ostringstream.members/str.move.pass.cpp

@@ -31,6 +31,12 @@ static void test() {
     assert(s == STR("testing"));
     assert(ss.view().empty());
   }
+  {
+    std::basic_ostringstream<CharT> ss;
+    std::basic_string<CharT> s = std::move(ss).str();
+    assert(s.empty());
+    assert(ss.view().empty());
+  }
 }
 
 int main(int, char**) {

libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.members/str.move.pass.cpp

@@ -31,6 +31,12 @@ static void test() {
     assert(s == STR("testing"));
     assert(buf.view().empty());
   }
+  {
+    std::basic_stringbuf<CharT> buf;
+    std::basic_string<CharT> s = std::move(buf).str();
+    assert(s.empty());
+    assert(buf.view().empty());
+  }
 }
 
 int main(int, char**) {

libcxx/test/std/input.output/string.streams/stringstream/stringstream.members/str.move.pass.cpp

@@ -31,6 +31,12 @@ static void test() {
     assert(s == STR("testing"));
     assert(ss.view().empty());
   }
+  {
+    std::basic_stringstream<CharT> ss;
+    std::basic_string<CharT> s = std::move(ss).str();
+    assert(s.empty());
+    assert(ss.view().empty());
+  }
 }
 
 int main(int, char**) {