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From: "hokein.wu at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/114903] New: constraint of CTAD alias deduction guide is
 evaluated on a wrong type
Date: Wed, 01 May 2024 08:06:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114903

            Bug ID: 114903
           Summary: constraint of CTAD alias deduction guide is evaluated
                    on a wrong type
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hokein.wu at gmail dot com
  Target Milestone: ---

See the following example of nested templates:=20=20

```
#include <type_traits>
template <typename T>
struct Key {
  Key(int);
};

class Forward {};

template <typename T>
constexpr bool C =3D
    std::is_same<T, Forward>(); // changing `Forward` to `double` will make=
 the
following static_assert passed.

template <typename Z>
struct Outer {
  template <typename U>
  struct Foo {
    Foo(U);
    U u;
  };

  template <typename V>
    requires(C<Z>)
  Foo(V) -> Foo<int>;
};

template <typename Y>
struct T {
  template <typename Y2>
  struct T2 {
    template <typename K>
    using AFoo =3D Outer<Y2>::template Foo<K>;
  };
};

T<Forward>::T2<Forward>::AFoo a{1.0};  // the explict deduction guide shoul=
d be
choosen, Foo<int>
static_assert(std::is_same<decltype(a), Outer<Forward>::Foo<int>>()); // ex=
pect
to be true!
```

The constraint `C` in the alias deduction guide should be true, as it is
evaluated on type `Forward`, but gcc seems to evaluate it on `double`.=