try making co_return strict

[bashtanov]

Avoid explicit constructors invocations, c-style conversions etc, stick to the same implicit conversions used for function calls and returns.

The reason I did not just static_assert for U being convertible to T but actually changed the way it is converted is because semantics of T(std::forward<U>(v)) may still be different from these of implicit cast in return v. This test would not pass without it:

struct wild_int {
    int value;
    wild_int(char c)
      : value(c - 10) {}
    explicit wild_int(int i)
      : value(i + 10) {}
};

struct c {
    static wild_int f() { return 42; }
    static ss::future<wild_int> g() { co_return 42; }
};

auto f_res = c::f();
auto g_res = c::g().get();
BOOST_CHECK_EQUAL(f_res.value, g_res.value);

(TODO add the test)

[bashtanov]

AFAIK placement new won't perform an excessive copy or move from an rvalue of the same type returned by id function.
This will need some performance testing.