<syncstream>: fix std::osyncstream memory leak

stl/inc/syncstream

@@ -218,11 +218,12 @@ protected:
         }
 
         const _Size_type _New_capacity  = _Calculate_growth(_Buf_size, _Buf_size + 1, _Max_allocation);
-        const _Elem* const _Old_ptr     = streambuf_type::pbase();
+        _Elem* const _Old_ptr           = streambuf_type::pbase();
         const _Size_type _Old_data_size = _Get_data_size();
 
         _Elem* const _New_ptr = _Unfancy(_Al.allocate(_New_capacity));
         _Traits::copy(_New_ptr, _Old_ptr, _Old_data_size);
+        _Al.deallocate(_Refancy<_Pointer>(_Old_ptr), _Old_data_size);
 
         streambuf_type::setp(_New_ptr, _New_ptr + _Old_data_size, _New_ptr + _New_capacity);
         streambuf_type::sputc(_Traits::to_char_type(_Current_elem));

tests/std/test.lst

@@ -204,6 +204,7 @@ tests\GH_002558_format_presetPadding
 tests\GH_002581_common_reference_workaround
 tests\GH_002655_alternate_name_broke_linker
 tests\GH_002711_Zc_alignedNew-
+tests\GH_002760_syncstream_memory_leak
 tests\LWG2597_complex_branch_cut
 tests\LWG3018_shared_ptr_function
 tests\LWG3121_constrained_tuple_forwarding_ctor

tests/std/tests/GH_002760_syncstream_memory_leak/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_20_matrix.lst

tests/std/tests/GH_002760_syncstream_memory_leak/test.cpp

@@ -0,0 +1,26 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// REQUIRES: debug_CRT
+
+#include <cassert>
+#include <cstdlib>
+#include <sstream>
+#include <syncstream>
+using namespace std;
+
+// GH-2760, <syncstream>: std::osyncstream memory leak
+int main() {
+    _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
+    [[maybe_unused]] _CrtMemState start;
+    [[maybe_unused]] _CrtMemState end;
+    [[maybe_unused]] _CrtMemState diff;
+    _CrtMemCheckpoint(&start);
+    {
+        stringstream s;
+        osyncstream bout(s);
+        bout << "Hello, this line is long, which previously leaked memory" << '\n';
+    }
+    _CrtMemCheckpoint(&end);
+    assert(_CrtMemDifference(&diff, &start, &end) == 0);
+}