Solidity: Uninitialised storage references should not be allowed

[axic]

Defining a struct instance within a function defaults to storage. This shouldn't be allowed.

contract c {
    struct s { uint a; }
    function f() {
        s x;
        x.a = 2;
    }
}

(From https://www.pivotaltracker.com/story/show/82613514.)

[chriseth]

We already have a warning here. The reason I did not turn this into an error was that there are use-cases where the warning can only be suppressed in a weird way:

contract c {
    struct s { uint a; }
    mapping(uint => s) data;
    function f(uint i) internal returns (s) {
        s x;
        if (i % 2 == 1)
          x = data[10 * i];
        else
           x = data[i / 2];
        return x;
    }
}

The far more dangerous situation where we do not have a warning yet (I guess I missed that) is:

contract c {
    struct s { uint a; }
    function f() internal returns (s) {
    }
}

[chriseth]

Storage references have to be assigned to at every point in the control flow they are used. This especially applies to functions returning storage pointers.

[chriseth]

'Use after free' is out of scope for this task.

[chriseth]

This has been fixed.

[microbecode]

Will this become an error in future releases? I'm unsure of what "this has been fixed" means.

[chriseth]

@microbecode yes, this will be a warning in future releases and an error starting from 0.5.0. Please try the nightly compiler builds if you want to check.

[sjmini]

@chriseth Will compiler throw error on below code from 0.5.0?

contract A {
   uint a = 0;

   function(){
      uint[] a;
      a[0] = 1;
   }
}

[chriseth]

@sjmini yes, multiple.