Re: Easy way to accept X and immutable X in parameters without overloading?

[Paul Backus via Digitalmars-d-learn]

On Monday, 11 January 2021 at 18:51:04 UTC, Jack wrote:
alias Callback = void function(const C, int);
void main()
{
    auto l = SList!Callback();
    auto a = (C c, int d) { };
    auto b = (C c, int d) { };
    auto c = (const C c, int d) { };
    l.insert(a);
    l.insert(b);
    l.insert(c);
}

You have a type mismatch. Changing the code to use explicit type 
annotations instead of `auto` makes the problem obvious:

alias Callback = void function(const C, int);

void main()
{
    Callback a = (C c, int d) { }; // Error
    Callback b = (C c, int d) { }; // Error
    Callback c = (const C c, int d) { };
}

The error message given is the same for both lines:

Error: cannot implicitly convert expression `__lambda1` of type 
`void function(C c, int d) pure nothrow @nogc @safe` to `void 
function(const(C), int)`

In other words, `a` and `b` are not valid Callbacks, because they 
take a mutable C argument instead of a const C argument.