libstdc++: Gracefully handle unrecognized _Arg_t values in basic_format_arg.

Implementing features from later revision of the standard, may require
extending the set of types stored directly in _Arg_value (without
changing in size and aliment), and thus expanding the values in _Arg_t.
However, any such value would be unrecognized by the TUs compiled with
older release, and would lead to UB (call to __builtin_unreachable).

This patch addresses above by introducing _M_handle_unrecognized method,
that is called instead. As specializations of this method for all
context supported at runtime (format_context and wformat_context) are
exported from libstdc++.so, the newest version (supporting all _Arg_t
values introduced later) will be picked (this is ensured by not providing
template definition in header in such case).
In consequence, the implementation may return handle object referencing
new alternatives (such wrapping is already required to provide standard
compliant behavior of visit_format_arg).
As no new _Arg_t were introduced since GCC16, this method simply throws
format_error now, and contains appropriate comment.

Note that the above is not required for formatting when __do_vformat_to
is exported (Unicode literal encoding and C++20). It remains necessary
in remaining cases or when visit on basic_format_arg is called by user.
In consequence addition of new argument types stored directly does not
have negative impact on performance in most common case.

This patch also removes the __type parameter from the _M_visit and
_M_visit_user, and uses _M_type member instead. This prevents misuse,
as the provided type was always required to match _M_type.

libstdc++-v3/ChangeLog:

	* config/abi/pre/gnu.ver (GLIBCXX_3.4.36): Export basic_format_arg
	_M_handle_unrecognized method of specializations of
	format_context and wformat_context.
	* include/std/format (basic_format_arg::_M_handle_unrecognized):
	Declare with explicit specializations for (w)format_context
	if _GLIBCXX_EXTERN_TEMPLATE, and define otherwise.
	(basic_format_arg::_M_visit): Remove __type parameter, and use
	_M_type instead. Call _M_handle_unrecognized for unrecognized
	_Arg_t values.
	(basic_format_arg::_M_visit_user): Remove __type parameter,
	and use _M_type instead. Adjust calls to _M_visit.
	(basic_format_arg::visit, std::visit_format_arg)
	(__format::__visit_format_arg): Remove _M_type argument from
	_M_visit(_user) calls.
	* src/c++20/format-inst.cc
	(basic_format_arg::_M_handle_unrecognized): Define and export
	explicit specializations for (w)format_context.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

diff --git a/libstdc++-v3/config/abi/pre/gnu.ver b/libstdc++-v3/config/abi/pre/gnu.ver
index 3a6afac830873ac1bcb69435833acfdd9678032e..9e1bebb4f500b107323c7430e111ea28abc33ade 100644 (file)
--- a/libstdc++-v3/config/abi/pre/gnu.ver
+++ b/libstdc++-v3/config/abi/pre/gnu.ver
@@ -2628,6 +2628,10 @@ GLIBCXX_3.4.36 {
 
     _ZNSt6chrono8__detail25__recent_leap_second_infoERNS_16leap_second_infoEj;
 
+    # basic_format_arg<format_context>::handle basic_format_arg<format_context>::_M_handle_unrecognized() const;
+    # basic_format_arg<wformat_context>::handle basic_format_arg<wformat_context>::_M_handle_unrecognized() const;
+    _ZNKSt16basic_format_argISt20basic_format_contextINSt8__format10_Sink_iterI[cw]EE[cw]EE22_M_handle_unrecognizedEv;
+
     # basic_string::allocate_at_least
     _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE*_S_allocate_*;
     _ZNSt7__cxx1112basic_stringI[cw]St11char_traitsI[cw]ESaI[cw]EE*_M_create_*;

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 8e7702d76df6bd3386d1fb6fc04881c7f1cf41af..1b4082e07a448403c618ccf125752b5005038923 100644 (file)
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -4404,12 +4404,12 @@ namespace __format
       template<typename _Visitor>
        _GLIBCXX_CONSTEXPR_FORMAT decltype(auto)
        visit(this basic_format_arg __arg, _Visitor&& __vis)
-       { return __arg._M_visit_user(std::forward<_Visitor>(__vis), __arg._M_type); }
+       { return __arg._M_visit_user(std::forward<_Visitor>(__vis)); }
 
       template<typename _Res, typename _Visitor>
        _GLIBCXX_CONSTEXPR_FORMAT _Res
        visit(this basic_format_arg __arg, _Visitor&& __vis)
-       { return __arg._M_visit_user(std::forward<_Visitor>(__vis), __arg._M_type); }
+       { return __arg._M_visit_user(std::forward<_Visitor>(__vis)); }
 #endif
 
     private:
@@ -4622,13 +4622,17 @@ namespace __format
        friend consteval __format::_Arg_t
        __format::__to_arg_t_enum() noexcept;
 
+      [[__gnu__::__noinline__]]
+      handle
+      _M_handle_unrecognized() const;
+
       template<typename _Visitor>
        _GLIBCXX_CONSTEXPR_FORMAT decltype(auto)
-       _M_visit(_Visitor&& __vis, __format::_Arg_t __type)
+       _M_visit(_Visitor&& __vis)
        {
-         using namespace __format;
-         switch (__type)
+         switch (_M_type)
          {
+           using enum __format::_Arg_t;
            case _Arg_none:
              return std::forward<_Visitor>(__vis)(_M_val._M_none);
            case _Arg_bool:
@@ -4693,13 +4697,17 @@ namespace __format
              return std::forward<_Visitor>(__vis)(_M_val._M_u128);
 #endif
            default:
-             __builtin_unreachable();
+             // Call exported definition of _M_handle_unrecognized from
+             // libstdc++.so, that should recognize new _Arg_t values and
+             // return basic_format_arg, containing a handle to that value.
+             handle __h = _M_handle_unrecognized();
+             return std::forward<_Visitor>(__vis)(__h);
          }
        }
 
       template<typename _Visitor>
        _GLIBCXX_CONSTEXPR_FORMAT decltype(auto)
-       _M_visit_user(_Visitor&& __vis, __format::_Arg_t __type)
+       _M_visit_user(_Visitor&& __vis)
        {
          return _M_visit([&__vis]<typename _Tp>(_Tp& __val) -> decltype(auto)
            {
@@ -4716,7 +4724,7 @@ namespace __format
                 handle __h(__val);
                 return std::forward<_Visitor>(__vis)(__h);
               }
-          }, __type);
+         });
        }
     };
 
@@ -4724,9 +4732,7 @@ namespace __format
     _GLIBCXX26_DEPRECATED_SUGGEST("std::basic_format_arg::visit")
     inline _GLIBCXX_CONSTEXPR_FORMAT decltype(auto)
     visit_format_arg(_Visitor&& __vis, basic_format_arg<_Context> __arg)
-    {
-      return __arg._M_visit_user(std::forward<_Visitor>(__vis), __arg._M_type);
-    }
+    { return __arg._M_visit_user(std::forward<_Visitor>(__vis)); }
 
 /// @cond undocumented
 namespace __format
@@ -4734,9 +4740,7 @@ namespace __format
   template<typename _Visitor, typename _Ctx>
     inline _GLIBCXX_CONSTEXPR_FORMAT decltype(auto)
     __visit_format_arg(_Visitor&& __vis, basic_format_arg<_Ctx> __arg)
-    {
-      return __arg._M_visit(std::forward<_Visitor>(__vis), __arg._M_type);
-    }
+    { return __arg._M_visit(std::forward<_Visitor>(__vis)); }
 
   struct _WidthPrecVisitor
   {
@@ -5069,6 +5073,26 @@ namespace __format
       advance_to(iterator __it) { _M_out = std::move(__it); }
     };
 
+#if _GLIBCXX_EXTERN_TEMPLATE
+   // The defintion _M_handle_unrecognized is placed in format-inst.cc
+   // source file, to ensure that it will not be inlined by compiler.
+   extern template basic_format_arg<format_context>::handle
+     basic_format_arg<format_context>::_M_handle_unrecognized() const;
+# ifdef _GLIBCXX_USE_WCHAR_T
+   extern template basic_format_arg<wformat_context>::handle
+     basic_format_arg<wformat_context>::_M_handle_unrecognized() const;
+# endif
+#else
+  template<typename _Context>
+    typename basic_format_arg<_Context>::handle
+    basic_format_arg<_Context>::_M_handle_unrecognized() const
+    {
+      // If _M_type corresponds to a new value of _Arg_t introduced after
+      // GCC 16, this function should return a handle that refers to the 
+      // union member of _M_val corresponding to that _Arg_t value.        
+      __throw_format_error("format error: unrecognized argument type");
+    }
+#endif
 
 /// @cond undocumented
 namespace __format

diff --git a/libstdc++-v3/src/c++20/format-inst.cc b/libstdc++-v3/src/c++20/format-inst.cc
index 65a3f2827327df2af43cef86e88f320422594ee4..fe15992485e578627e30afb977b4ce8360a615bc 100644 (file)
--- a/libstdc++-v3/src/c++20/format-inst.cc
+++ b/libstdc++-v3/src/c++20/format-inst.cc
@@ -27,6 +27,25 @@
 namespace std
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  template<typename _Context>
+    typename basic_format_arg<_Context>::handle
+    basic_format_arg<_Context>::_M_handle_unrecognized() const
+    {
+      // If _M_type corresponds to a new value of _Arg_t introduced after GCC 16,
+      // this function should return a handle that refers to the union member of
+      // _M_val corresponding to that _Arg_t value.        
+      __throw_format_error("format error: unrecognized argument type");
+    }
+
+ template basic_format_arg<format_context>::handle
+   basic_format_arg<format_context>::_M_handle_unrecognized() const;
+
+# ifdef _GLIBCXX_USE_WCHAR_T
+ template basic_format_arg<wformat_context>::handle
+   basic_format_arg<wformat_context>::_M_handle_unrecognized() const;
+# endif
+
 namespace __format
 {