Not initialized out argument error

[bearophile via Digitalmars-d]

D has plenty of semantic annotations, but sometimes they are not 
handled in a strict enough way, so their usefulness (in catching 
bugs) is reduced.

Microsoft catches plenty of troubles statically with SAL2:
http://msdn.microsoft.com/en-us/library/hh916382.aspx

In this program D gives an error:


class Foo {
     immutable int[] x;
     this() {}
}
void main() {}


test.d(3,5): Error: constructor test.Foo.this missing initializer 
for immutable field x


D could give a similar error here, an out argument that is not 
assigned inside the function "foo":

void foo(out int x) {}
void main() {}


A possible error message:

test.d(1,21): Error: uninitialised out argument of 'test3.foo' 
function


This analysis is basic, so this code does not raise that error, 
despite arr[1] is not initialized:

void foo(out int[2] arr) {
     arr[0] = 1;
}


Another case where the compiler will not give that 'uninitialised 
out argument' error, despite arr is not always initialized:

void foo(in bool b, out int[2] arr) {
     if (b)
         arr = [1, 2];
}


When and if D language and compiler adds some form of flow 
analysis, the analysis for out could become more strict, and 
refuse the precedent two examples.

This is a breaking change, so if you want you can reach this 
error with a warning-deprecation-error trajectory.

I think that most D code with functions with out arguments that 
are not initialized inside the function are already buggy so I 
think this code will not introduce lot of breakage in existing D 
code.

Bye,
bearophile