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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/82833] [concepts] Out-of-line definition of nested class
 template errors with constraint (ICE)
Date: Mon, 19 Jul 2021 10:04:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D82833

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Looks like the ICE was fixed by the concepts rewrite in r10-3735 "Update the
concepts implementation to conform to C++20."


Valid C++20 testcase:

#include <functional>
#include <concepts>

template <typename T, auto A, typename M>
concept CallYieldsType =3D requires(T t) {
  { std::invoke(A, t) } -> std::same_as<M>;
};

template <typename T>
struct S {
  struct Z;


  template <auto A>
    requires CallYieldsType<T, A, Z>
  struct N;
};

template <typename T>
template <auto A>
  requires CallYieldsType<T, A, typename S<T>::Z>
struct S<T>::N {
  Z &f(T t) {
    return std::invoke(A, t);
  }
};


Bisections shows this started to be accepted with r11-3713:

    c++: Set the constraints of a class type sooner [PR96229]

    In the testcase below, during processing (at parse time) of Y's base
    class X<Y>, convert_template_argument calls is_compatible_template_arg
    to check if the template argument Y is no more constrained than the
    parameter P.  But at this point we haven't yet set Y's constraints, so
    get_normalized_constraints_from_decl yields NULL_TREE as the normal form
    and caches this result into the normalized_map.

    We set Y's constraints later in cp_parser_class_specifier_1 but the
    stale normal form in the normalized_map remains.  This ultimately causes
    us to miss the constraint failure for Y<Z> because according to the
    cached normal form, Y is not constrained.

    This patch fixes this issue by moving up the call to
    associate_classtype_constraints so that we set constraints before we
    start processing a class's bases.

    gcc/cp/ChangeLog:

            PR c++/96229
            * parser.c (cp_parser_class_specifier_1): Move call to
            associate_classtype_constraints from here to ...
            (cp_parser_class_head): ... here.
            * pt.c (is_compatible_template_arg): Correct documentation to
            say "argument is _no_ more constrained than the parameter".

    gcc/testsuite/ChangeLog:

            PR c++/96229
            * g++.dg/cpp2a/concepts-class2.C: New test.=