re PR libstdc++/51795 (linear_congruential_engine doesn't work correctly)

2012-01-27  Paolo Carlini  <paolo.carlini@oracle.com>

	PR libstdc++/51795
	* include/bits/random.h (linear_congruential_generator): Add
	static_assert preventing instantiation for values of 'a' and 'm'
	currently handled incorrectly but _Mod::__calc.
	* include/bits/random.tcc (seed_seq::generate): Avoid unsafe
	uses of _Mod::__calc.

From-SVN: r183655