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Assertions were originally enabled in the compiled-in floating-point std::to_chars implementation to help shake out any bugs, but they apparently impose a significant performance penalty, most notably for the hex formatting which is around 25% slower with assertions enabled. This seems too high a cost for unconditionally enabling them. The newly added calls to __builtin_unreachable work around the compiler no longer knowing that the set of valid values of 'fmt' is limited (which was previously upheld by an assert). libstdc++-v3/ChangeLog: * src/c++17/floating_to_chars.cc (_GLIBCXX_ASSERTIONS): Don't define. (__floating_to_chars_shortest): Add __builtin_unreachable calls to squelch false-positive -Wmaybe-uninitialized and -Wreturn-type warnings. (__floating_to_chars_precision): Likewise.
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