libstdc++: Fix non-portable results from 64-bit std::subtract_with_carry_engine [PR107466]

I implemented the resolution of LWG 3809 in r13-4364-ga64775a0edd469 but
it was recently noted in the MSVC STL github repo that the change causes
possible truncation for 64-bit seeds. Whether the truncation occurs (and
to what value) depends on the width of uint_least32_t which is not
portable, so the output of the PRNG for 64-bit seed values is no longer
the same as in C++20, and no longer portable across platforms.

That new issue was filed as LWG 4014. I proposed a new change which
reduces the seed by the LCG's modulus before the conversion to
uint_least32_t. This ensures that 64-bit seed values are consistently
reduced by the modulus before any truncation. This removes the
platform-dependent behaviour and restores the old behaviour for
std::subtract_with_carry_engine specializations using a 64-bit result
type (such as std::ranlux48_base).

libstdc++-v3/ChangeLog:

	PR libstdc++/107466
	* include/bits/random.tcc (subtract_with_carry_engine::seed):
	Implement proposed resolution of LWG 4014.
	* testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
	line number.
	* testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc:
	Check for expected result of 64-bit engine with seed that
	doesn't fit in 32-bits.

0 files changed, 0 insertions, 0 deletions