The fallback implementation of floating-point std::from_chars for e.g. float80 just calls the C library's strtod family of functions. In case of overflow of the parsed result, the behavior of these functions is rigidly specified:
If the correct value overflows and default rounding is in effect, plus or minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the return type and sign of the value), and the value of the macro ERANGE is stored in errno. But in case of underflow, implementations are given more leeway: If the result underflows the functions return a value whose magnitude is no greater than the smallest normalized positive number in the return type; whether errno acquires the value ERANGE is implementation-defined. Thus we can (and do) portably detect overflow, but we can't portably detect underflow. However, glibc (and presumably other high-quality C library implementations) will reliably set errno to ERANGE in case of underflow too, and it will also return the nearest denormal number to the parsed result (including zero in case of true underflow). Since we can't be perfect here, this patch takes the best effort approach of assuming a high quality C library implementation that allows us to distinguish between a denormal parsed result and true underflow by inspecting the return value. Tested on x86_64-pc-linux-gnu, does this look OK for trunk? Dunno if we should backport this too. No test because we can't portably test this IIUC. libstdc++-v3/ChangeLog: * src/c++17/floating_from_chars.cc (from_chars_impl): In the ERANGE case, also check for a 0 return value before returning result_out_of_range, occurred, otherwise assume it's a denormal number. --- libstdc++-v3/src/c++17/floating_from_chars.cc | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc b/libstdc++-v3/src/c++17/floating_from_chars.cc index a25ac5ce3aa..939c751f861 100644 --- a/libstdc++-v3/src/c++17/floating_from_chars.cc +++ b/libstdc++-v3/src/c++17/floating_from_chars.cc @@ -637,8 +637,13 @@ namespace { if (__builtin_isinf(tmpval)) // overflow ec = errc::result_out_of_range; - else // underflow (LWG 3081 wants to set value = tmpval here) + else if (tmpval == 0) // underflow (LWG 3081 wants to set value = tmpval here) ec = errc::result_out_of_range; + else // denormal value + { + value = tmpval; + ec = errc(); + } } else if (n) { -- 2.38.1.381.gc03801e19c