fix writing NaN to the output stream with a set locale

stl/inc/xlocnum

@@ -1376,7 +1376,7 @@ protected:
         const auto _Ngen = static_cast<size_t>(_CSTD sprintf_s(
             &_Buf[0], _Buf.size(), _Ffmt(_Fmt, 0, _Iosbase.flags()), static_cast<int>(_Precision), _Val));
 
-        return _Fput(_Dest, _Iosbase, _Fill, _Buf.c_str(), _Ngen);
+        return _Fput_v2(_Dest, _Iosbase, _Fill, _Buf.c_str(), _Ngen, (_STD isnan)(_Val));
     }
 
     virtual _OutIt __CLR_OR_THIS_CALL do_put(
@@ -1400,7 +1400,7 @@ protected:
         const auto _Ngen = static_cast<size_t>(_CSTD sprintf_s(
             &_Buf[0], _Buf.size(), _Ffmt(_Fmt, 'L', _Iosbase.flags()), static_cast<int>(_Precision), _Val));
 
-        return _Fput(_Dest, _Iosbase, _Fill, _Buf.c_str(), _Ngen);
+        return _Fput_v2(_Dest, _Iosbase, _Fill, _Buf.c_str(), _Ngen, (_STD isnan)(_Val));
     }
 #pragma warning(pop)
 
@@ -1462,7 +1462,13 @@ private:
     }
 
     _OutIt __CLRCALL_OR_CDECL _Fput(_OutIt _Dest, ios_base& _Iosbase, _Elem _Fill, const char* _Buf,
-        size_t _Count) const { // put formatted floating-point to _Dest
+        size_t _Count) const { // TRANSITION, ABI: preserved for binary compatibility
+        return _Fput_v2(_Dest, _Iosbase, _Fill, _Buf, _Count, false);
+    }
+
+    template <int = 0> // TRANSITION, ABI
+    _OutIt _Fput_v2(_OutIt _Dest, ios_base& _Iosbase, _Elem _Fill, const char* _Buf, size_t _Count,
+        bool _Is_nan_val) const { // put formatted floating-point to _Dest
         auto _Prefix = static_cast<size_t>(0 < _Count && (*_Buf == '+' || *_Buf == '-'));
         const char* _Exps;
         if ((_Iosbase.flags() & ios_base::floatfield) != ios_base::hexfloat) {
@@ -1491,13 +1497,15 @@ private:
             _Groupstring[_Poff] = _Punct_fac.decimal_point();
         }
 
-        size_t _Off     = _Poff == _Count ? _Eoff : _Poff;
-        const char* _Pg = &_Grouping[0];
-        while (*_Pg != CHAR_MAX && '\0' < *_Pg && static_cast<size_t>(*_Pg) < _Off - _Prefix) {
-            // add thousands separator
-            _Groupstring.insert(_Off -= *_Pg, 1, _Kseparator);
-            if ('\0' < _Pg[1]) {
-                ++_Pg; // not last group, advance
+        if (!_Is_nan_val) {
+            size_t _Off     = _Poff == _Count ? _Eoff : _Poff;
+            const char* _Pg = &_Grouping[0];
+            while (*_Pg != CHAR_MAX && '\0' < *_Pg && static_cast<size_t>(*_Pg) < _Off - _Prefix) {
+                // add thousands separator
+                _Groupstring.insert(_Off -= *_Pg, 1, _Kseparator);
+                if ('\0' < _Pg[1]) {
+                    ++_Pg; // not last group, advance
+                }
             }
         }
 

tests/std/test.lst

@@ -227,6 +227,7 @@ tests\GH_003617_vectorized_meow_element
 tests\GH_003676_format_large_hh_mm_ss_values
 tests\GH_003735_char_traits_signatures
 tests\GH_003840_tellg_when_reading_lf_file_in_text_mode
+tests\GH_003867_output_nan
 tests\LWG2381_num_get_floating_point
 tests\LWG2597_complex_branch_cut
 tests\LWG3018_shared_ptr_function

tests/std/tests/GH_003867_output_nan/env.lst

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

tests/std/tests/GH_003867_output_nan/test.cpp

@@ -0,0 +1,24 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <limits>
+#include <locale>
+#include <sstream>
+
+using namespace std;
+
+template <typename T>
+void test_gh_3867() {
+    // GH-3867 Writing NaN to the output stream with a set locale results in a weird output
+    ostringstream s;
+    s.imbue(locale("en-US"));
+    s << -numeric_limits<T>::quiet_NaN();
+    assert(s.str() == "-nan(ind)");
+}
+
+int main() {
+    test_gh_3867<float>();
+    test_gh_3867<double>();
+    test_gh_3867<long double>();
+}