[cfe-dev] UBSan false positive (?) with virtual inheritance

[Alexey Sidorin via cfe-dev]

Hello everyone,

The code example below triggers an undefined behavior sanitizer warning 
when compiled with -O1 and higher:

      1  struct Aux {
      2    virtual ~Aux() = default;
      3    int i = 0;
      4  };
      5
      6  struct Base {
      7    virtual ~Base() = default;
      8  };
      9
     10  struct A : public virtual Base, public Aux {};
     11
     12  struct B final : public virtual A {};
     13
     14  void check(const A &a) {}
     15
     16  int main() {
     17    B b;
     18    check(b);// UBSan warns here
     19    return 0;
     20  }

When compiled with -fsanitize=undefined:

example.cpp:18:9: runtime error: reference binding to address 
0x7ffe652149c8 with insufficient space for an object of type 'const A'

When compiled with -fsanitize=object-size:

example.cpp:12:8: runtime error: constructor call on address 
0x7ffc7988a2e0 with insufficient space for an object of type 'A'

example.cpp:18:9: runtime error: reference binding to address 
0x7ffc7988a2e0 with insufficient space for an object of type 'const A'

Could someone tell if it is a sanitizer false positive or does this case 
actually contains a kind of UB? GCC doesn't give any warning for this code.


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