Weird uninitialized const member behavior

Consider this code snippet:

struct Foo {

};

template<typename T>
struct Bar {
    const T foo;
};

int main() {
    Bar<Foo> test;
}

I'm compiling it with g++-4.9.2 with [-std=c++11] and getting error: uninitialized const member in ‘struct Bar<Foo>’. This is pretty obvious.

BUT try just add std::string as Foo member and program compiles!

#include <string>
struct Foo {
    std::string test;
};
// (...)

What happening? Replacing test's type into double causes program to fail to compile again. What string member changes in class?