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### 2927. Encoding a functor and argument types as a function signature for `is_callable` and `result_of` is fragile

Section: 21.3.3 [meta.type.synop], 21.3.7 [meta.rel] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-21

Priority: Not Prioritized

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Discussion:

Addresses US 85

The trick of encoding a functor and argument types as a function signature for `is_callable` and `result_of` loses cv information on argument types, fails for non-decayed function types, and is confusing. E.g.,

>     typedef int MyClass::*mp; result_of_t<mp(const MyClass)>;
>     // should be const, but isn't
>     typedef int F(double); is_callable<F(float)>; // ill-formed

Minimal change:

Replace `is_callable<Fn(ArgTypes...)>` with `is_callable<Fn, ArgTypes...>` and replace `is_callable<Fn(ArgTypes...), R>` with `is_callable_r<R, Fn, ArgTypes...>`. Do the same for `is_nothrow_callable`.

Preferred change:

All of the above, plus `deprecate result_of<Fn(ArgTypes...)>` and replace it with `result_of_invoke<Fn, ArgTypes...>`

See also LWG 2895.

[2017-02-22, Daniel comments]

LWG 2895 provides now wording for this issue and for LWG 2928 as well.

[2017-02-24, Daniel comments]

I suggest to apply the paper d0604r0 instead, available on the Kona LWG wiki.

[2017-03-12, post-Kona]

Resolved by p0604r0.

Proposed resolution: