Re: DIP66 - Multiple alias this

[IgorStepanov via Digitalmars-d]

On Friday, 10 October 2014 at 20:47:45 UTC, Steven Schveighoffer 
wrote:
On 10/10/14 1:09 PM, IgorStepanov wrote:
I've created DIP for my pull request.
DIP: http://wiki.dlang.org/DIP66
PR: https://github.com/D-Programming-Language/dmd/pull/3998
Please, comment it.

This part:

void test()
    {
        C c;
        int i = c; //Error: c.a.i vs c.b.i
    }

    static assert(is(C : int)); //Ok, because C is subtype of 
int anyway.

I think might be wrong. There is a lot of code out there that 
says, e.g.:

void foo(T)(T t) if(is(T : U))
{
  U u = t;
  ...
}

Which will now create an error in the wrong place. IMO, the 
'is' test should also fail.

-Steve

I thought exactly about this using case.

See:
You have a struct like this in first place:
    struct A
    {
        int i;
        alias i this;
    }

    struct C
    {
        A a;
        string s;
        alias a this;
        alias s this;
    }

And you have a template function in second place:
void foo(T)(T t) if(is(T : int))
{
...
}

void foo(T)(T t) if(is(T : string))
{
...
}


And you have the code it third place:
C c;
foo(c); //Error: what do you mean: foo!(T : string) or foo!(T : 
int)

Now, someone (A developer) changed the A definition:

    struct A
    {
        int i;
        alias i this;
    }

    struct B
    {
        int i;
        alias i this;
    }

    struct C
    {
        A a;
        B b;
        string s;
        alias a this;
        alias b this;
        alias s this;
    }

And now, you code mystically start to works.
Attention: Infusing in one place conflict resolves conflict in 
another place.
It is danger, I think.