fix formatting floating-point values without type and precision provided (#4327)

Author: fsb4000

stl/inc/format

@@ -2967,6 +2967,7 @@ _NODISCARD _OutputIt _Fmt_write(
     auto _Format    = chars_format::general;
     auto _Exponent  = 'e';
     auto _Precision = _Specs._Precision;
+    auto _None_type = false;
 
     switch (_Specs._Type) {
     case 'A':
@@ -3003,6 +3004,9 @@ _NODISCARD _OutputIt _Fmt_write(
         }
         _Format = chars_format::general;
         break;
+    default:
+        _None_type = true;
+        break;
     }
 
     // Consider the powers of 2 in decimal:
@@ -3043,7 +3047,11 @@ _NODISCARD _OutputIt _Fmt_write(
         _Result.ptr += 3;
     } else {
         if (_Precision == -1) {
-            _Result = _STD to_chars(_Buffer, _STD end(_Buffer), _Value, _Format);
+            if (_None_type) {
+                _Result = _STD to_chars(_Buffer, _STD end(_Buffer), _Value);
+            } else {
+                _Result = _STD to_chars(_Buffer, _STD end(_Buffer), _Value, _Format);
+            }
         } else {
             _Result = _STD to_chars(_Buffer, _STD end(_Buffer), _Value, _Format, _Precision);
         }

tests/std/tests/P0645R10_text_formatting_formatting/test.cpp

@@ -1506,6 +1506,12 @@ constexpr bool test_format_string() {
     return true;
 }
 
+// Also test GH-4319: incorrect output for some floating-point values
+template <class charT>
+void test_gh_4319() {
+    assert(format(STR("{:}"), 12345678.0) == STR("12345678"));
+}
+
 void test() {
     test_simple_formatting<char>();
     test_simple_formatting<wchar_t>();
@@ -1582,6 +1588,9 @@ void test() {
     test_localized_char<char, char>();
     test_localized_char<wchar_t, char>();
     test_localized_char<wchar_t, wchar_t>();
+
+    test_gh_4319<char>();
+    test_gh_4319<wchar_t>();
 }
 
 int main() {