Pfthb

method arrayFromSeq<T(0)> (s: seq<T>) returns (a:array<T>)
 ensures a[..] == s
 ensures fresh(a)
]; 
//forall i 

I would like to substitute the first two lines in the body by the third one, in order to avoid the qualifier (0) in type T, but it raises an "index out of range" error.

You can use this modified version of the third line instead

a := new T[|s|] (i requires 0 <= i < |s| => s[i]);

The reason that your version of the third line doesn't work is that Dafny verifies anonymous functions separately from their context. By looking at the function i => s[i] in isolation, Dafny is worried that i might be out of bounds.

The fix is to introduce a precondition to this anonymous function, which is what I showed above. Now, when looking at the function in isolation, the precondition guarantees that the index will be in bounds.

Separately, Dafny has to check that this precondition is satisfied by the use of the function. This check passes, because Dafny knows that array initialization new T[|s|] will only call the function on arguments that are between 0 and |s|.