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From: "gcc-90 at tbilles dot hu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95693] New: Incorrect error from undefined behavior sanitizer
Date: Tue, 16 Jun 2020 05:12:23 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 10.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95693

            Bug ID: 95693
           Summary: Incorrect error from undefined behavior sanitizer
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc-90 at tbilles dot hu
  Target Milestone: ---

After upgrading GCC from 7.2 to 10.1 I get a runtime error when using
-fsanitize=3Dundefined. I cannot see anything wrong with the code. It was a=
lso
suggested on the gcc-help mailing list that this is a bug in the compiler.
(https://gcc.gnu.org/pipermail/gcc-help/2020-June/139055.html)

The code in question can be found on Compiler Explorer (also pasted at the =
end
of this description): https://godbolt.org/z/7rAxJj

It shows that different compiler versions behave differently.

Version 10.1 prints "runtime error: reference binding to null pointer of ty=
pe
'int'" although there is no null pointer in the code, the reference is boun=
d to
a global integer.

There is a comment on line 16 that explicitly defaults the Derived construc=
tor.
If you switch the comment with line 17, both compiler versions run fine wit=
hout
producing the runtime error although the defaulted constructor should be
exactly the same the user defined one: https://godbolt.org/z/UShm-u

According to Compiler Explorer the incorrect behavior began in GCC 8.x seri=
es
and is still present in trunk.

---
int global =3D 9;

class Payload {
public:
    Payload() : data(global) {}
private:
    int& data;
};

struct Base {
    Payload payload;
};

class Derived : public Base {
public:
    //Derived() =3D default;
    Derived() : Base() {}

    Payload p;
};

int main()
{
    Derived t;
}=