<xloctime>: time_get::get can still assert 'istreambuf_iterator is not dereferenceable' when the format is longer than the stream (#2851)

stl/inc/xloctime

@@ -575,7 +575,10 @@ protected:
         const _Ctype& _Ctype_fac = _STD use_facet<_Ctype>(_Iosbase.getloc());
 
         for (; *_Fmtfirst != '\0'; ++_Fmtfirst) {
-            if (*_Fmtfirst == '%') {
+            if (_First == _Last) {
+                _State |= ios_base::failbit;
+                break;
+            } else if (*_Fmtfirst == '%') {
                 _First = do_get(_First, _Last, _Iosbase, _State, _Pt,
                     *++_Fmtfirst); // convert a single field
             } else if (*_Fmtfirst == ' ') {

tests/std/tests/Dev11_0836436_get_time/test.cpp

@@ -109,6 +109,7 @@ void test_locale_chinese();
 void test_invalid_argument();
 void test_buffer_resizing();
 void test_gh_2618();
+void test_gh_2848();
 
 int main() {
     assert(read_hour("12 AM") == 0);
@@ -155,6 +156,7 @@ int main() {
     test_invalid_argument();
     test_buffer_resizing();
     test_gh_2618();
+    test_gh_2848();
 }
 
 typedef istreambuf_iterator<char> Iter;
@@ -873,3 +875,33 @@ void test_gh_2618() {
     // 1-digit strings: same as 2-digit
     TestTimeGetYear("1", 2001, 1, 2001);
 }
+
+void test_gh_2848() {
+    // GH-2848 <xloctime>: time_get::get can still assert 'istreambuf_iterator is not dereferenceable' when
+    // the format is longer than the stream
+    {
+        const locale loc{locale::classic()};
+        const auto& tmget{use_facet<time_get<char>>(loc)};
+        ios_base::iostate err{ios_base::goodbit};
+        tm when{};
+        const string fmt{"%X"};
+        istringstream iss{"3:04"};
+        istreambuf_iterator<char> first{iss};
+        const istreambuf_iterator<char> last{};
+        tmget.get(first, last, iss, err, &when, fmt.data(), fmt.data() + fmt.size());
+        assert(err == (ios_base::eofbit | ios_base::failbit));
+    }
+
+    {
+        const locale loc{locale::classic()};
+        const auto& tmget{use_facet<time_get<wchar_t>>(loc)};
+        ios_base::iostate err{ios_base::goodbit};
+        tm when{};
+        const wstring fmt{L"%X"};
+        wistringstream iss{L"3:04"};
+        istreambuf_iterator<wchar_t> first{iss};
+        const istreambuf_iterator<wchar_t> last{};
+        tmget.get(first, last, iss, err, &when, fmt.data(), fmt.data() + fmt.size());
+        assert(err == (ios_base::eofbit | ios_base::failbit));
+    }
+}