The floating-point overloads of from_char are only declared if
_GLIBCXX_HAVE_USELOCALE is #defined as nonzero. That's exposed from
charconv as __cpp_lib_to_chars >= 201611L, so guard the test body with
that.
for libstdc++-v3/ChangeLog
PR c++/105324
* testsuite/20_util/from_chars/pr105324.cc: Guard test body
with conditional for floating-point overloads of from_char.
int main()
{
+#if __cpp_lib_to_chars >= 201611L // FP from_char not available otherwise.
// PR libstdc++/105324
// std::from_chars() assertion at floating_from_chars.cc:78
std::string s(512, '1');
s[1] = '.';
long double d;
std::from_chars(s.data(), s.data() + s.size(), d);
+#endif
}