<random>: urbg.seed(urbg) does not compile

[strega-nil-ms]

For every URBG in the standard library, I believe the following should work:

#include <random>

int main() {
    std::linear_congruential_engine<unsigned, 48271U, 0U, 2147483647U> rng1, rng2;
    rng1.seed(rng2);
}

This is due to _Enable_if_seed_seq_t being overspecified - it excludes the type itself from overload resolution, which makes sense for the constructor (since we want the copy constructor to win), but does not make sense for seed.

(note that this works):

#include <random>

int main() {
    std::linear_congruential_engine<unsigned, 48271U, 0U, 2147483647U> rng1;
    std::linear_congruential_engine<unsigned, 48271U, 1U, 2147483647U> rng2;
    rng1.seed(rng2);
}

discovered while investigating #2693

[CaseyCarter]

The Standard ([tab:rand.req.eng] rows 5-7) says that seed must accept no argument, a result_type value, or an argument that meets the seed sequence requirements. Generally speaking, URBGs don't meet the seed sequence requirements.

TR1 (I'm looking at N1836 Table 16 rows 4-6) says that seed must accept no arguments, a result_type value, or "an lvalue of a type other than [the type of the engine that seed is a member of] that defines a zero-argument function object returning values of unsigned integral type".

So I think this behavior is by design.

[StephanTLavavej]

We looked at this during the weekly maintainer meeting and determined that @CaseyCarter is correct - the seed sequence requirements involve many member functions such as generate() that the engines don't provide.