Fix C++20 incompatibility from using exception specifications.

C++20 removed support for `throw()` exception specifications, so at
least clang-19 is now emitting errors on them. Replaced exception
specifications with BOOST_NOEXCEPT_OR_NOTHROW, which converts `throw()`
to `noexcept`, but keeps the code formally compatible with C++03.

Author: Lastique

include/boost/program_options/errors.hpp

@@ -121,7 +121,7 @@ namespace boost { namespace program_options {
         /** gcc says that throw specification on dtor is loosened 
          *  without this line                                     
          *  */ 
-        BOOST_DEFAULTED_FUNCTION(~error_with_option_name() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~error_with_option_name() BOOST_NOEXCEPT_OR_NOTHROW, {})
 
 
         //void dump() const
@@ -183,7 +183,7 @@ namespace boost { namespace program_options {
 
         /** Creates the error_message on the fly
          *      Currently a thin wrapper for substitute_placeholders() */
-        virtual const char* what() const throw();
+        virtual const char* what() const BOOST_NOEXCEPT_OR_NOTHROW;
 
     protected:
         /** Used to hold the error text returned by what() */
@@ -209,7 +209,7 @@ namespace boost { namespace program_options {
         multiple_values() 
          : error_with_option_name("option '%canonical_option%' only takes a single argument"){}
 
-        BOOST_DEFAULTED_FUNCTION(~multiple_values() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~multiple_values() BOOST_NOEXCEPT_OR_NOTHROW, {})
     };
 
     /** Class thrown when there are several occurrences of an
@@ -220,7 +220,7 @@ namespace boost { namespace program_options {
         multiple_occurrences() 
          : error_with_option_name("option '%canonical_option%' cannot be specified more than once"){}
 
-        BOOST_DEFAULTED_FUNCTION(~multiple_occurrences() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~multiple_occurrences() BOOST_NOEXCEPT_OR_NOTHROW, {})
 
     };
 
@@ -233,7 +233,7 @@ namespace boost { namespace program_options {
        {
        }
 
-       BOOST_DEFAULTED_FUNCTION(~required_option() throw(), {})
+       BOOST_DEFAULTED_FUNCTION(~required_option() BOOST_NOEXCEPT_OR_NOTHROW, {})
     };
 
     /** Base class of unparsable options,
@@ -258,7 +258,7 @@ namespace boost { namespace program_options {
         /** Does NOT set option name, because no option name makes sense */
         virtual void set_option_name(const std::string&) {}
 
-        BOOST_DEFAULTED_FUNCTION(~error_with_no_option_name() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~error_with_no_option_name() BOOST_NOEXCEPT_OR_NOTHROW, {})
     };
 
 
@@ -270,7 +270,7 @@ namespace boost { namespace program_options {
         {
         }
 
-        BOOST_DEFAULTED_FUNCTION(~unknown_option() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~unknown_option() BOOST_NOEXCEPT_OR_NOTHROW, {})
     };
 
 
@@ -283,9 +283,9 @@ namespace boost { namespace program_options {
             m_alternatives(xalternatives)
         {}
 
-        BOOST_DEFAULTED_FUNCTION(~ambiguous_option() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~ambiguous_option() BOOST_NOEXCEPT_OR_NOTHROW, {})
 
-        const std::vector<std::string>& alternatives() const throw() {return m_alternatives;}
+        const std::vector<std::string>& alternatives() const BOOST_NOEXCEPT_OR_NOTHROW {return m_alternatives;}
 
     protected:
         /** Makes all substitutions using the template */
@@ -320,7 +320,7 @@ namespace boost { namespace program_options {
         {
         }
 
-        BOOST_DEFAULTED_FUNCTION(~invalid_syntax() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~invalid_syntax() BOOST_NOEXCEPT_OR_NOTHROW, {})
 
         kind_t kind() const {return m_kind;}
 
@@ -340,7 +340,7 @@ namespace boost { namespace program_options {
             m_substitutions["invalid_line"] = invalid_line;
         }
 
-        BOOST_DEFAULTED_FUNCTION(~invalid_config_file_syntax() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~invalid_config_file_syntax() BOOST_NOEXCEPT_OR_NOTHROW, {})
 
         /** Convenience functions for backwards compatibility */
         virtual std::string tokens() const {return m_substitutions.find("invalid_line")->second;    }
@@ -355,7 +355,7 @@ namespace boost { namespace program_options {
                        const std::string& original_token = "",
                        int option_style              = 0):
             invalid_syntax(kind, option_name, original_token, option_style) {}
-        BOOST_DEFAULTED_FUNCTION(~invalid_command_line_syntax() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~invalid_command_line_syntax() BOOST_NOEXCEPT_OR_NOTHROW, {})
     };
 
 
@@ -380,7 +380,7 @@ namespace boost { namespace program_options {
         {
         }
 
-        BOOST_DEFAULTED_FUNCTION(~validation_error() throw(), {})
+        BOOST_DEFAULTED_FUNCTION(~validation_error() BOOST_NOEXCEPT_OR_NOTHROW, {})
 
         kind_t kind() const { return m_kind; }
 

src/value_semantic.cpp

@@ -260,7 +260,7 @@ namespace boost { namespace program_options {
     }
 
 
-    const char* error_with_option_name::what() const throw()
+    const char* error_with_option_name::what() const BOOST_NOEXCEPT_OR_NOTHROW
     {
         // will substitute tokens each time what is run()
         substitute_placeholders(m_error_template);