The Property Functions documentation reads:

Is this warning still valid? The standard library appears to extensively use property functions.

On Sunday, 16 May 2021 at 15:12:25 UTC, Nick wrote:

The @property thing doesn't do much. All it does is change typeof(a.prop) from function over to the return value of the function. (Which actually makes it required for the range empty and front things!)

But since it doesn't do much it makes it very easy to misuse it too - putting it somewhere where it doesn't belong will NOT cause the compiler to issue an error.

So that's why it is warned: the current behavior is extremely minimal and if that expands and you misused it, you'll see broken code down the line.

But on the other hand, @property has been a do-nothing for a decade now, so I wouldn't expect that to change any time soon.

My general rule is to put it on something that should be replaceable with a public data member. If you can't do that, don't make it @property. In particular, do NOT put it on something for the sole reason of not using () on the call. @property is not really related to parenthesis syntax. Only use it when it is specifically meant to be replaceable with a public member.

On Sunday, 16 May 2021 at 15:47:55 UTC, Adam D. Ruppe wrote:

It's not required:

struct Example
{
    int front() { return 42; }
    bool empty() { return false; }
    void popFront() {}
}

import std.range;
static assert(isInputRange!Example);

On Sunday, 16 May 2021 at 15:47:55 UTC, Adam D. Ruppe wrote:

Thanks for the detailed explanation. I guess the wording of the warning seems strange to me, in that it recommends not to use property functions; but, in actuality (and regardless?), these functions are used a lot (even in the standard library; at least from what I've seen). Also, although this warning is delivered, there is nothing in the documentation that explicitly addresses a concrete downside (or recommended limitation) to using them. However, your explanation does provide some context to this warning.