https://gcc.gnu.org/g:4fd42fce8d43cefb59c61d2865c198c35c0860be
commit r17-2118-g4fd42fce8d43cefb59c61d2865c198c35c0860be Author: Tomasz Kamiński <[email protected]> Date: Wed Jun 24 11:40:08 2026 +0200 libstdc++: Provide defined behavior for unrecognized _Pres_type values. If the new _Pres_type values are introduced for given type, they may lead to unrecognized _Pres_type values, if the TU using them is linked with TU compiled with older releases, and format from old TU is selected. For most of the formatters, the default implementation is used as the fallback, however __formatter_int and __formatter_fp were treating that as UB, due to call to __builtin_unreachable in default branch the switch. This patch addresses above by fallbacking to _Pres_none behavior in such case. Note that this for C++20 affects programs using non-Unicode literal encoding, as __do_vformat_to is exported from the library otherwise, and thus newest version is always picked. libstdc++-v3/ChangeLog: * include/std/format (__formatter_int::format) (__formatter_fp::format): For unrecognised _M_spec._M_type values (default branch of switch) fallthrou to _Pres_none. Reviewed-by: Jonathan Wakely <[email protected]> Signed-off-by: Tomasz Kamiński <[email protected]> Diff: --- libstdc++-v3/include/std/format | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format index db226ecb0c84..872a86c76a4a 100644 --- a/libstdc++-v3/include/std/format +++ b/libstdc++-v3/include/std/format @@ -1722,6 +1722,7 @@ namespace __format case _Pres_c: return _M_format_character(_S_to_character(__i), __fc); #endif + default: // Fallback for _Pres_type values introduces in later versions. case _Pres_none: // Should not reach here with _Pres_none for bool or charT, so: [[fallthrough]]; @@ -1741,8 +1742,6 @@ namespace __format for (auto __p = __start; __p != __res.ptr; ++__p) *__p = __format::__toupper_numeric(*__p); break; - default: - __builtin_unreachable(); } if (_M_spec._M_alt && __base_prefix.size()) @@ -2172,12 +2171,11 @@ namespace __format __use_prec = true; __fmt = chars_format::general; break; + default: // Fallback for _Pres_type values introduces in later versions. case _Pres_none: if (__use_prec) __fmt = chars_format::general; break; - default: - __builtin_unreachable(); } // Write value into buffer using std::to_chars.
