TYPE_NEEDS_CONSTRUCTING zero for std::string?

[Martin Sebor via Gcc]

I'd expect TYPE_NEEDS_CONSTRUCTING to be non-zero in the middle end
for any C++ type with a user-defined ctor, but in some of my testing
I see it's actually zero for std::string, at least in some instances
(but nonzero for other types with ctors).  Is there something special
about std::string that makes it so?  (If this is intentional, how can
I create my own type that has a user-defined ctor and also a zero
TYPE_NEEDS_CONSTRUCTING?)

Thanks
Martin