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From: "dangelog at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102396] [11/12 Regression] ICE when using concepts, in get, 
 at cp/constraint.cc:2637 since r11-6245-g79f57d5cb070bb02
Date: Thu, 09 Dec 2021 18:54:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102396

--- Comment #4 from Giuseppe D'Angelo <dangelog at gmail dot com> ---
To elaborate on the last comment, this testcase does complain about the
redefinition.=20


    #include <type_traits>
    #include <concepts>


    template <typename T> class S;

    template <typename T>
    static inline std::true_type is_S_impl(const S<T>&);
    static inline std::false_type is_S_impl(...);

    template <typename T>
    concept is_S =3D decltype(is_S_impl(std::declval<T>()))::value;



    template <typename T>
    struct S {
        using type =3D T;
        T data;

        S();
        explicit S(const T &);

        template <typename V, typename U>
        requires is_S<V> && (!is_S<U>) &&
std::equality_comparable_with<typename V::type, U>
        friend bool operator=3D=3D(const V &v, const U &u) { return v.data =
=3D=3D u; }
    };

    void f() {
        S<int> s1; s1 =3D=3D 123;
        S<double> s2; s2 =3D=3D 123.4;
    }=