I'm curious what exactly makes this undefined behavior.

And in particular, what about something like this?

    struct Foo {
    #ifdef __cplusplus
      int bar() const { return bar_; }
     private:
    #endif
      int bar_;
    };

Or, taking this a step further:

    struct _Foo;
    typedef struct _Foo Foo;

    // In C "struct _Foo" is never defined.
    int Foo_bar(const Foo* foo) { return *(int*)foo; }
    void Foo_setbar(Foo* foo) { *(int*)foo; }
    Foo* Foo_new() { return malloc(sizeof(int)); }

    #ifdef __cplusplus
    struct _Foo {
      void set_bar() { bar_ = bar; }
      int bar() const { return bar_; }
     private:
      int bar_;
    };
    #endif

The above isn't ideal but it does provide encapsulation in a way that doesn't seem to violate strict aliasing (the memory location is consistently read/written as "int").