I feel like I must be missing something here.
This works:
----
alias T = int;
T** f(const T** input) pure
{
T** output;
return output;
}
void main()
{
T i;
T* p = &i;
immutable T** r = f(&p);
}
----
`f` is `pure`, its parameter is const, and its return type has mutable
indirections. That makes it a "pure factory function" [1].
Since `f` is a pure factory function, the compiler can assume that the
result is not referenced from anywhere else. So I can declare it
`immutable`.
So far, so good.
Now change `T` to `alias T = immutable int;`. The program gets rejected.
The error message is: "cannot implicitly convert expression (f(& p)) of
type immutable(int)** to immutable(int**)".
What changed? `f` can now return a reference to `i`. But that's not a
problem, because that part of the return type is already `immutable`.
What would be a problem is if `f` could return a reference to `p`. But
it can't, as far as I can tell.
Am I missing something or could/should the program be accepted with `T =
immutable int`? What could `f` do that would break `r`'s immutability?
[1] https://dlang.org/spec/function.html#pure-functions
