isCallable fails for template opCall overload

[dlangBugzillaToGithub]

snarwin+bugzilla (@pbackus) reported this on 2019-09-26T18:50:31Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=20246

CC List

• maxsamukha
• simen.kjaras
• zan77137

Description

import std.traits: isCallable;

struct S { int opCall(T)(T) { return 0; } }

static assert(isCallable!(S.init));

---

The above static assert should pass, but instead fails.

[dlangBugzillaToGithub]

simen.kjaras commented on 2019-09-27T06:34:43Z

It's impossible to distinguish between the above case and this:

struct S { enum opCall(T) = true; }

Clearly, the latter is *not* callable.

To illustrate the situation perhaps more clearly:

struct S {
    template opCall(T) {
        static if (is(T == int)) {
            void opCall(T t) {}
        } else {
            enum opCall = 14;
        }
    }
}

The above is perfectly valid code, but should isCallable!S return true?

[dlangBugzillaToGithub]

snarwin+bugzilla commented on 2019-09-27T15:35:26Z

Seems like this is an issue with the interface, then, not the implementation; the question "is S.init callable" is undecidable, so there's not much use trying to pretend we can answer it.

One possible solution would be a trait like `isCallableWith!(S.init, int)`, that requires the user to specify the argument type(s). That way, we could just check with __traits(compiles) whether the call is valid or not.

[dlangBugzillaToGithub]

maxsamukha commented on 2019-09-28T06:27:27Z

(In reply to Paul Backus from comment #2)

> One possible solution would be a trait like `isCallableWith!(S.init, int)`,
> that requires the user to specify the argument type(s). That way, we could
> just check with __traits(compiles) whether the call is valid or not.

That does not take into account rvalue-ness of the arguments:

void foo(ref int) {}
store
static if (isCallableWith!(typeof(1)))
    foo(1); // error

isCallableWith would have to accept the actual arguments, (type, rvalue-ness) pairs, or both.

And if one implements isCallableWith for functions, he should also implement isInstantiatableWith for templates. And that is a case for general __traits(matchFunction/matchTemplate, symbol, args) that would return the overloads callable/instantiatable with args. isCallableWith/isInstantiatableWith could be built on that.

[dlangBugzillaToGithub]

maxsamukha commented on 2019-09-28T06:29:30Z

(In reply to Max Samukha from comment #3)

> isCallableWith!(typeof(1))

Typo: isCallableWith!(foo, typeof(1))

[dlangBugzillaToGithub]

snarwin+bugzilla commented on 2019-09-28T15:30:04Z

(In reply to Max Samukha from comment #3)
> That does not take into account rvalue-ness of the arguments:
> 
> void foo(ref int) {}
> store
> static if (isCallableWith!(typeof(1)))
>     foo(1); // error
> 
> isCallableWith would have to accept the actual arguments, (type,
> rvalue-ness) pairs, or both.

Then perhaps the best solution is to add a note to the documentation of isCallable highlighting its shortcomings, and encourage everyone to instead use __traits(compiles) directly with the actual function call they want to make.

[dlangBugzillaToGithub]

simen.kjaras commented on 2020-08-10T07:26:58Z

*** Issue 21137 has been marked as a duplicate of this issue. ***

[dlangBugzillaToGithub]

simen.kjaras commented on 2020-08-10T07:40:37Z

It's impossible to solve this in the general case, and even the special cases can't be solved in library code, but a __traits could check if the only member of the template is a callable (potentially also if all possible members would be callables).