Merge pull request #1413 from boostorg/1412

<sstream> is not needed in exception free environment

Author: mborland

include/boost/math/policies/error_handling.hpp

@@ -26,9 +26,9 @@
 #include <complex>
 #include <cstdint>
 #include <cstring>
+#ifndef BOOST_MATH_NO_EXCEPTIONS
 #include <iomanip>
 #include <sstream>
-#ifndef BOOST_MATH_NO_EXCEPTIONS
 #include <stdexcept>
 #endif
 #include <string>
@@ -95,6 +95,8 @@ T user_indeterminate_result_error(const char* function, const char* message, con
 namespace detail
 {
 
+#ifndef BOOST_MATH_NO_EXCEPTIONS
+// Only used to build exception messages; relies on <sstream>
 template <class T>
 inline std::string prec_format(const T& val)
 {
@@ -124,7 +126,9 @@ inline std::string prec_format<std::float128_t>(const std::float128_t& val)
    return std::string(buffer, r.ptr);
 }
 
-#endif
+#endif // BOOST_MATH_USE_CHARCONV_FOR_CONVERSION
+
+#endif // BOOST_MATH_NO_EXCEPTIONS
 
 inline void replace_all_in_string(std::string& result, const char* what, const char* with)
 {

test/Jamfile.v2

@@ -202,6 +202,7 @@ test-suite special_fun :
    [ run git_issue_1255.cpp ]
    [ run git_issue_1247.cpp ]
    [ run git_issue_1308.cpp /boost/test//boost_unit_test_framework ]
+   [ run git_issue_1412.cpp ]
    [ run special_functions_test.cpp /boost/test//boost_unit_test_framework  ]
    [ run test_airy.cpp test_instances//test_instances pch_light /boost/test//boost_unit_test_framework  ]
    [ run test_bessel_j.cpp test_instances//test_instances pch_light /boost/test//boost_unit_test_framework  ]

test/git_issue_1412.cpp

@@ -0,0 +1,23 @@
+//  Copyright Matt Borland 2026
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Regression test for https://github.com/boostorg/math/issues/1412
+
+#define BOOST_MATH_NO_EXCEPTIONS
+
+// Claim the <iomanip> include guard so any later #include <iomanip> expands to nothing and std::setprecision is unavailable.
+// The original reproducer with _SSTREAM broke <complex> which iso outside our purview
+#define _LIBCPP_IOMANIP        // libc++
+#define _GLIBCXX_IOMANIP 1     // libstdc++
+
+#include <boost/math/policies/error_handling.hpp>
+#include <boost/math/special_functions/laguerre.hpp>
+
+int main()
+{
+    // Exercise a special function so the no-exceptions header path is
+    // instantiated, not just parsed.
+    return static_cast<int>(boost::math::laguerre(1u, 0u, 0.5));
+}