Make STL components not dependent on is_trivial(_v)
#5202
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Towards #5137/WG21-P3247R2. IMO most related changes should be done for older modes. libstdc++ is already doing so.
Product code
basic_string: There requirements seem to be slightly strengthened, but I don't believe a "character-like type" with a private default constructor can ever be supported.basic_string_view, it seems that "character-like type" with a private default constructor can be supported. I'm not sure whether it should be changed._Can_memcpy_val: Trivial default-constructibility is not actually requried, so I just relax the condition tois_trivially_copyable_v._Uninitialized_copy_multidimensional(_al): Like above, but the dispatching condition should be more precise._Circular_buffer:static_assertdoesn't seem necessary as the involved types are either scalar or implementation-controlled, but the checks seem desired to avoid regression._Unaligned_load: The function template is used and instantiated only once, so I think it's better to inline it into the caller.Test code
is_trivially_copyable_v&is_trivially_default_constructible_v" is slight stronger than "is_trivial_v", I decide to use the former style thoroughly except whenis_trivial(_v)themselves are tested.