concepts

[ZombineDev via Digitalmars-d]

On Thursday, 21 May 2015 at 11:05:10 UTC, Dennis Ritchie wrote:
> Is it possible to write something like this in the compile time?
>
> auto foo(T)(T t)
>     if (is(T == immutable(int)) && t == 5)
>     // Error: variable t cannot be read at compile time
> {
>     // ...
> }
>
> void main()
> {
>     immutable(int) n = 5;
>     foo(n);
> }

Although the compiler already has enough information, this is not 
possible because the template constraints are not allowed to use 
the runtime parameters (they are compile-time only constraints). 
It would be more doable like this:

auto foo(T)(T t)
     if (is(T == immutable(int)))
{
     assert(t == 5);
}

void main()
{
     immutable int n = 6;
     foo(n); // Possible in the future:
     // dmd test.d --deep-asserts-verification
     // Error: function foo has an assert which can't satisfied:
     // assert(5 == n) <- n is 6
}