<format>: crash when formatting a floating-point value with #/L and a large precision (#4907)

Author: Andor233

stl/inc/format

@@ -3095,7 +3095,7 @@ _NODISCARD _OutputIt _Fmt_write(
         switch (_Format) {
         case chars_format::hex:
         case chars_format::scientific:
-            if (_Extra_precision != 0) {
+            if (_Extra_precision != 0 && !_Specs._Alt && !_Specs._Localized) {
                 // Trailing zeroes are in front of the exponent
                 while (*--_Exponent_start != _Exponent) {
                 }

tests/std/tests/P0645R10_text_formatting_formatting/test.cpp

@@ -1520,6 +1520,26 @@ void test_gh_4319() {
     assert(format(STR("{:}"), 12345678.0) == STR("12345678"));
 }
 
+// GH-4320: <format>: crash when formatting a floating-point value with #/L and a large precision
+template <class charT>
+void test_gh_4320() {
+    assert(
+        format(STR("{:#.1075e}"), 1.0)
+        == STR(
+            "1."
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+            "00000000000000000000000000000000000e+00"));
+}
+
 void test() {
     test_simple_formatting<char>();
     test_simple_formatting<wchar_t>();
@@ -1604,6 +1624,8 @@ void test() {
 
     test_gh_4319<char>();
     test_gh_4319<wchar_t>();
+    test_gh_4320<char>();
+    test_gh_4320<wchar_t>();
 }
 
 int main() {