Move-assignment selects the wrong assignment operator

[jeffgarrett]

The greedy assignment operator is missing a constraint against assigning from the same type.

using FR = tl::function_ref<void(void)>;
static_assert(std::is_trivially_assignable<FR, FR&>::value, "");       // fails
static_assert(std::is_trivially_assignable<FR, const FR&>::value, ""); // OK
static_assert(std::is_trivially_assignable<FR, FR&&>::value, "");      // fails

The constraint is in P0792r5.