const data member triggers dynamic initialization with C++ compiler by biochimia

Below is a reduced test case exemplifying the issue.

    // main.cpp
    #include <assert.h>

    struct Data { int ref; };
    struct ConstData { const Data header; }; // < problematic const

    extern ConstData sharedNull;

    struct File {
        File() { assert(sharedNull.header.ref == -1); }
    };

    File global;
    ConstData sharedNull = { { -1 } };

    int main() {}

Removing the const in the commented line produces the expected results below, but I don't see why that should trigger zero-initialization followed by dynamic-initialization of the global sharedNull.

Changing the const-ness of the global sharedNull variable doesn't affect the results either way. The issue has also been reproduced in optimized builds.