Akira,

it prevents type deduction of template parameter U.

see also
https://en.cppreference.com/w/cpp/types/type_identity

with a better example to show where it is used to prevent conflicts in
argument deduction.

However, I cannot answer your second question for sure.

Regards
Peter.

Akira Takahashi via Std-Discussion wrote on 18.12.20 07:04:
> Hi,
> I have a question.
>
> P1021R3 Filling holes in Class Template Argument Deduction
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1021r3.html
>
> P1814R0 Wording for Class Template Argument Deduction for Alias Templates
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1814r0.html
>
> template <class T, class U>
> struct C {
> C(T, U); // #1
> };
>
> template <class T, class U>
> C(T, U) -> C<T, std::type_identity_t<U>>; #2 THIS
>
> What's the usage of type_identity?
> Why not directly use a constructor?
>
> Thanks,
> Akira
>
>

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Peter Sommerlad
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