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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/88162] GCC does not accept non-type template parameters of
 class type
Date: Mon, 23 Aug 2021 02:13:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D88162

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
ICC also rejects this at both -std=3Dc++17 and -std=3Dc++20:
<source>(15): error: a nontype template parameter may not have class type
  template<template<auto> class T> using nttp_t =3D typename decltype(
f(T<zero>()) )::type;
                                                                         ^

<source>(15): error: no instance of function template "f" matches the argum=
ent
list
            argument types are: (int_constant<<error-constant>>)
  template<template<auto> class T> using nttp_t =3D typename decltype(
f(T<zero>()) )::type;
                                                                     ^
<source>(6): note: this candidate was rejected because at least one template
argument could not be deduced
  template<auto X, template<auto> class T> id<decltype(X)> f( T<X> )
                                                           ^
          detected during instantiation of type "nttp_t<int_constant>" at l=
ine
22

<source>(15): error: no instance of function template "f" matches the argum=
ent
list
            argument types are: (char_constant<<error-constant>>)
  template<template<auto> class T> using nttp_t =3D typename decltype(
f(T<zero>()) )::type;
                                                                     ^
<source>(6): note: this candidate was rejected because at least one template
argument could not be deduced
  template<auto X, template<auto> class T> id<decltype(X)> f( T<X> )
                                                           ^
          detected during instantiation of type "nttp_t<char_constant>" at =
line
22

<source>(15): error: no instance of function template "f" matches the argum=
ent
list
            argument types are: (long_constant<<error-constant>>)
  template<template<auto> class T> using nttp_t =3D typename decltype(
f(T<zero>()) )::type;
                                                                     ^
<source>(6): note: this candidate was rejected because at least one template
argument could not be deduced
  template<auto X, template<auto> class T> id<decltype(X)> f( T<X> )
                                                           ^
          detected during instantiation of type "nttp_t<long_constant>" at =
line
22

<source>(15): error: no instance of function template "f" matches the argum=
ent
list
            argument types are: (voidp_constant<<error-constant>>)
  template<template<auto> class T> using nttp_t =3D typename decltype(
f(T<zero>()) )::type;
                                                                     ^
<source>(6): note: this candidate was rejected because at least one template
argument could not be deduced
  template<auto X, template<auto> class T> id<decltype(X)> f( T<X> )
                                                           ^
          detected during instantiation of type "nttp_t<voidp_constant>" at
line 22

compilation aborted for <source> (code 2)=