Fix missing negative sign with precision 0

include/boost/charconv/detail/dragonbox/floff.hpp

@@ -1764,7 +1764,7 @@ BOOST_CHARCONV_SAFEBUFFERS to_chars_result floff(const double x, int precision,
 
             const auto initial_digits = static_cast<std::uint32_t>(prod >> 32);
 
-            buffer -= (initial_digits < 10 && buffer != first ? 1 : 0);
+            buffer -= (initial_digits < 10 && buffer != first && precision != 0 ? 1 : 0);
             remaining_digits -= (2 - (initial_digits < 10 ? 1 : 0));
 
             // Avoid the situation where we have a leading 0 that we don't need

include/boost/charconv/detail/ryu/ryu_generic_128.hpp

@@ -436,14 +436,18 @@ static inline int generic_to_chars_fixed(const struct floating_decimal_128 v, ch
             {
                 current_len = static_cast<int>(shift_width) + precision;
             }
+            else
+            {
+                current_len = static_cast<int>(shift_width) - 1;
+            }
 
             precision = 0;
-            // Since we wrote additional characters into the buffer we need to add a null terminator,
+            // Since we wrote additional characters into the buffer, we need to add a null terminator,
             // so they are not read
             const auto round_val = result[current_len];
             result[current_len] = '\0';
 
-            // More complicated rounding situations like 9999.999999 are already handled
+            // More complicated rounding situations like 9999.999999 are already handled,
             // so we don't need to worry about rounding past the decimal point
             if (round_val >= '5')
             {

test/Jamfile

@@ -75,3 +75,4 @@ run github_issue_267.cpp ;
 run github_issue_280.cpp ;
 run github_issue_282.cpp ;
 run github_issue_int128_320.cpp ;
+run github_issue_296.cpp ;

test/github_issue_296.cpp

@@ -0,0 +1,67 @@
+// Copyright 2026 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+//
+// See: https://github.com/boostorg/charconv/issues/296
+
+#include <boost/charconv.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename T>
+void test()
+{
+    const T value = -125.125;
+
+    char buffer[64];
+    const auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), value, boost::charconv::chars_format::fixed, 0);
+    BOOST_TEST(r);
+    *r.ptr = '\0';
+
+    BOOST_TEST_CSTR_EQ(buffer, "-125");
+}
+
+template <typename T>
+void test_pos()
+{
+    const T value = 125.125;
+
+    char buffer[64];
+    const auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), value, boost::charconv::chars_format::fixed, 0);
+    BOOST_TEST(r);
+    *r.ptr = '\0';
+
+    BOOST_TEST_CSTR_EQ(buffer, "125");
+}
+
+template <typename T>
+void test_prec_1()
+{
+    const T value = -125.125;
+
+    char buffer[64];
+    const auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), value, boost::charconv::chars_format::fixed, 1);
+    BOOST_TEST(r);
+    *r.ptr = '\0';
+
+    BOOST_TEST_CSTR_EQ(buffer, "-125.1");
+}
+
+int main()
+{
+    test<float>();
+    test<double>();
+
+    test_pos<float>();
+    test_pos<double>();
+
+    test_prec_1<float>();
+    test_prec_1<double>();
+
+    #if !defined(BOOST_CHARCONV_UNSUPPORTED_LONG_DOUBLE) && BOOST_CHARCONV_LDBL_BITS < 128
+    test<long double>();
+    test_pos<long double>();
+    test_prec_1<long double>();
+    #endif
+
+    return boost::report_errors();
+}