libstdc++: Disable broken std::format for floating-point types [PR108221]

If we don't have std::to_chars for floating-point types (either because
float and double are not IEEE format, or size_t is 16-bit) then we can't
use them with std::format. This causes a bootstrap failure since
std/c++20/tzdb.cc was added to the library, because <chrono> now
includes <format>.

This change just disables formatting support for those types. This is
not a proper fix, but solves the bootstrap failure for now.

libstdc++-v3/ChangeLog:

	PR libstdc++/108221
	* include/std/format (basic_format_arg) [!__cpp_lib_to_chars]:
	Disable visiting floating-point types.

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 98421e8c123e4808f7eb52241a87e12aa5b177bd..77f7c9fef3f149d29e7b72cebf24e1d53695db02 100644 (file)
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -3034,6 +3034,7 @@ namespace __format
              return std::forward<_Visitor>(__vis)(_M_val._M_ll);
            case _Arg_ull:
              return std::forward<_Visitor>(__vis)(_M_val._M_ull);
+#if __cpp_lib_to_chars // FIXME: need to be able to format these types!
            case _Arg_flt:
              return std::forward<_Visitor>(__vis)(_M_val._M_flt);
            case _Arg_dbl:
@@ -3046,6 +3047,7 @@ namespace __format
              return std::forward<_Visitor>(__vis)(_M_val._M_f128);
            case _Arg_ibm128:
              return std::forward<_Visitor>(__vis)(_M_val._M_ibm128);
+#endif
 #endif
            case _Arg_str:
              return std::forward<_Visitor>(__vis)(_M_val._M_str);