Add test set from issue

Author: mborland

test/Jamfile

@@ -70,4 +70,5 @@ run github_issue_166.cpp ;
 run github_issue_166_float128.cpp ;
 run github_issue_186.cpp ;
 run github_issue_212.cpp ;
+run github_issue_266.cpp ;
 run github_issue_267.cpp ;

test/github_issue_266.cpp

@@ -0,0 +1,35 @@
+// Copyright 2025 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/266
+
+#include <boost/charconv.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename T>
+void test(T value, int precision, const char* correct_result)
+{
+    char buffer[64] {};
+    const auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), value, boost::charconv::chars_format::fixed, precision);
+    BOOST_TEST_CSTR_EQ(buffer, correct_result);
+    BOOST_TEST(r);
+}
+
+int main()
+{
+    test(123456789012345.6789012345678901, 5, "123456789012345.67188");
+    test(1234567890123456.789012345678901, 5, "1234567890123456.75000");
+    test(12345678901234567.89012345678901, 5, "12345678901234568.00000");
+    test(123456789012345678.9012345678901, 5, "123456789012345680.00000");
+    test(1234567890123456789.012345678901, 5, "1234567890123456768.00000");
+
+    test(123456789012345678901234567.8901, 5, "123456789012345678152597504.00000");
+    test(12345678901234567890123456.78901, 5, "12345678901234568244756480.00000");
+    test(1234567890123456789012345.678901, 5, "1234567890123456824475648.00000");
+    test(123456789012345678901234.5678901, 5, "123456789012345685803008.00000");
+    test(12345678901234567890123.45678901, 5, "12345678901234567741440.00000");
+    test(123456789012345678901.2345678901, 5, "123456789012345683968.00000");
+
+    return boost::report_errors();
+}