sqrt for cpp_int_backend behaves incorrect for small argument values

[Rüdiger Brünner <rbruenner@…>]

The sqrt function for multiprecision integers does not work for small values. Using this code:

checked_int128_t int1;
for (int i = 10; i >= 0; i--)
{
	int1 = i;
	try
	{
		int1 = sqrt(int1);
		std::cout << "sqrt(" << i << ") = " << int1 << std::endl;
	}
	catch (std::exception& ex)
	{
		std::cout << "sqrt(" << i << ") -> " << typeid(ex).name() << ": " << ex.what() << std::endl;
	}
}

I get the following output:

sqrt(10) = 3
sqrt(9) = 3
sqrt(8) = 2
sqrt(7) = 2
sqrt(6) = 2
sqrt(5) = 2
sqrt(4) = 2
sqrt(3) -> class boost::exception_detail::clone_impl<struct boost::exception_detail::error_info_injector<class std::overflow_error> >: Unable to allocate sufficient storage for the value of the result: value overflows the maximum allowable magnitude.
sqrt(2) -> class boost::exception_detail::clone_impl<struct boost::exception_detail::error_info_injector<class std::overflow_error> >: Unable to allocate sufficient storage for the value of the result: value overflows the maximum allowable magnitude.
sqrt(1) = 0
sqrt(0) = 0

While sqrt(3) and sqrt(2) are raising an exception, sqrt(1) gives a wrong result.

[John Maddock]

Fixed in ​https://github.com/boostorg/multiprecision/commit/4538e88a91b47a72a3e136672d1a7389e9035fd8

Thanks for the report!