# V765. Compound assignment expression 'X += X + N' is suspicious. Consider inspecting it for a possible error.

The analyzer detected a potential error in an arithmetic or logical expression: a variable is used both in the left and the right parts of a compound-assignment expression.

Consider the following example:

    void Foo(int x, int y, int z)
    {
      x += x + y;
      ....
    }

This code is likely to contain a typo and was probably meant to look like this:

    void Foo(int x, int y, int z)
    {
      x = x + y;
      ....
    }

Or like this:

    void Foo(int x, int y, int z)
    {
      x += z + y;
      ....
    }

It is true that programmers use expressions like these as a tricky means to multiply a number by two, but such code is strange and needs to be checked. Such expressions look rather complicated and probably should be rewritten in a simpler and clearer way:

    void Foo(int x, int y, int z)
    {
      x = x * 2 + y;
      ....
    }

There are also more suspicious expressions that need to be inspected:

    void Foo(int x, int y)
    {
      x -= x + y;
    }

This expression can be simplified in the following way:

- x -= x + y;
- x = x - (x + y);
- x = -y;

It is not clear if this behavior is intended or caused by a typo. In any case, this code should be checked.

This diagnostic is classified as:

CWE-682

You can look at examples of errors detected by the V765 diagnostic.