Redundant declaration errors from Boost Assert compiling with g++ -Werror -Wredundant-decls -DBOOST_ENABLE_ASSERT_HANDLER

[jleffler]

Consider the (minimal) source file redundant.cpp (yes, just two #include lines)

#include "boost/shared_ptr.hpp"
#include "boost/shared_array.hpp"

Consider the shell script that compiles that — where there is a copy of Boost 1.81.0 in the sub-directory boost_1_81_0 of the current directory:

#!/bin/sh

source=redundant.cpp
: "${boostdir:=./boost_1_81_0}"

set -x
g++ -c -Werror -Wredundant-decls -DBOOST_ENABLE_ASSERT_HANDLER \
    -I"$boostdir" "$source"

Because of the two -W and one -D options, the code fails to compile, with errors:

+ g++ -c -Werror -Wredundant-decls -DBOOST_ENABLE_ASSERT_HANDLER -I./boost_1_81_0 redundant.cpp
In file included from ./boost_1_81_0/boost/smart_ptr/shared_array.hpp:21,
                 from ./boost_1_81_0/boost/shared_array.hpp:17,
                 from redundant.cpp:2:
./boost_1_81_0/boost/assert.hpp:49:10: error: redundant redeclaration of ‘void boost::assertion_failed(const char*, const char*, const char*, long int)’ in same scope [-Werror=redundant-decls]
   49 |     void assertion_failed(char const * expr, char const * function, char const * file, long line); // user defined
      |          ^~~~~~~~~~~~~~~~
In file included from ./boost_1_81_0/boost/smart_ptr/shared_ptr.hpp:24,
                 from ./boost_1_81_0/boost/shared_ptr.hpp:17,
                 from redundant.cpp:1:
./boost_1_81_0/boost/assert.hpp:49:10: note: previous declaration of ‘void boost::assertion_failed(const char*, const char*, const char*, long int)’
   49 |     void assertion_failed(char const * expr, char const * function, char const * file, long line); // user defined
      |          ^~~~~~~~~~~~~~~~
In file included from ./boost_1_81_0/boost/smart_ptr/shared_array.hpp:21,
                 from ./boost_1_81_0/boost/shared_array.hpp:17,
                 from redundant.cpp:2:
./boost_1_81_0/boost/assert.hpp:50:10: error: redundant redeclaration of ‘void boost::assertion_failed_msg(const char*, const char*, const char*, const char*, long int)’ in same scope [-Werror=redundant-decls]
   50 |     void assertion_failed_msg(char const * expr, char const * msg, char const * function, char const * file, long line); // user defined
      |          ^~~~~~~~~~~~~~~~~~~~
In file included from ./boost_1_81_0/boost/smart_ptr/shared_ptr.hpp:24,
                 from ./boost_1_81_0/boost/shared_ptr.hpp:17,
                 from redundant.cpp:1:
./boost_1_81_0/boost/assert.hpp:50:10: note: previous declaration of ‘void boost::assertion_failed_msg(const char*, const char*, const char*, const char*, long int)’
   50 |     void assertion_failed_msg(char const * expr, char const * msg, char const * function, char const * file, long line); // user defined
      |          ^~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

The file (boost_1_81_0/boost/assert.hpp) can be fixed easily by adding a guard around the declaration of the two functions:

$ diff -u boost_1_81_0/boost/assert.hpp boost_1_81_0/boost/assert-20230317.142448.hpp 
--- boost_1_81_0/boost/assert.hpp	2023-03-17 15:12:31.136740000 -0600
+++ boost_1_81_0/boost/assert-20230317.142448.hpp	2023-03-17 14:24:48.699552000 -0600
@@ -44,11 +44,15 @@
 #include <boost/config.hpp> // for BOOST_LIKELY
 #include <boost/current_function.hpp>
 
+// Avoid warnings from -Wredundant-decls
+#ifndef BOOST_ASSERT_DECLARE_ASSERTION_FAILED_FUNCTIONS
+#define BOOST_ASSERT_DECLARE_ASSERTION_FAILED_FUNCTIONS
 namespace boost
 {
     void assertion_failed(char const * expr, char const * function, char const * file, long line); // user defined
     void assertion_failed_msg(char const * expr, char const * msg, char const * function, char const * file, long line); // user defined
 } // namespace boost
+#endif
 
 #define BOOST_ASSERT(expr) (BOOST_LIKELY(!!(expr))? ((void)0): ::boost::assertion_failed(#expr, BOOST_CURRENT_FUNCTION, __FILE__, __LINE__))
 #define BOOST_ASSERT_MSG(expr, msg) (BOOST_LIKELY(!!(expr))? ((void)0): ::boost::assertion_failed_msg(#expr, msg, BOOST_CURRENT_FUNCTION, __FILE__, __LINE__))
$

I don't know if the comment would be retained in the final version of the code. And I've chosen what seemed like a plausible macro name for protecting the code, but I'm perfectly happy with any equivalent choice. With this fix in place, the file redundant.cpp compiles without errors. Of course, it does nothing useful, but that's what happens when you minimize the code that reproduces the problem.