[Bug c++/82833] New: [concepts] Out-of-line definition of nested class template errors with constraint (ICE)

[kjcamann.lists at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82833

            Bug ID: 82833
           Summary: [concepts] Out-of-line definition of nested class
                    template errors with constraint (ICE)
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kjcamann.lists at gmail dot com
  Target Milestone: ---

The behavior is different between gcc 7.1 and my latest gcc 8 build. gcc 7
produces an error message, whereas in gcc 8 (built from a1920064e2, around
11/3/2017) it is an ICE.

Here is the code:

```
#include <functional>

// Check that std::invoke(<any constexpr callable>, <instance of t>)
// yields an `M` type
template <typename T, auto A, typename M>
concept bool CallYieldsType = requires(T t) {
  { std::invoke(A, t) } -> M;
};

template <typename T>
struct S {
  struct Z;

  // Declared here, but defined out of line. If defined here, this works.
  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);
  }
};
```

This error is related to the combination of concept checking and out-of-line
definition, because if I remove either one (by removing the requires clause,
*or* by defining S<T>::N inline) it works fine.

gcc 7.1 says:

test3.cpp:29:16: error: ‘S<T>::N<A>’ does not match any declaration
 struct S<T>::N {

gcc 8 says:

test3.cpp:29:16: error: ‘S<T>::N<A>’ does not match any declaration
 struct S<T>::N {
                ^
test3.cpp:29:16: internal compiler error: tree check: expected class ‘type’,
have ‘exceptional’ (error_mark) in cp_parser_class_specifier_1, at
cp/parser.c:22492
0x6868c8 tree_class_check_failed(tree_node const*, tree_code_class, char
const*, int, char const*)
        ../.././gcc/tree.c:9146
0x85085c tree_class_check(tree_node*, tree_code_class, char const*, int, char
const*)
        ../.././gcc/tree.h:3211
0x85085c cp_parser_class_specifier_1
        ../.././gcc/cp/parser.c:22492
0x850ae1 cp_parser_class_specifier
        ../.././gcc/cp/parser.c:22754
0x850ae1 cp_parser_type_specifier
        ../.././gcc/cp/parser.c:16818
0x85fc31 cp_parser_decl_specifier_seq
        ../.././gcc/cp/parser.c:13684
0x864d25 cp_parser_single_declaration
        ../.././gcc/cp/parser.c:27131
0x8650bc cp_parser_template_declaration_after_parameters
        ../.././gcc/cp/parser.c:26823
0x865b4c cp_parser_explicit_template_declaration
        ../.././gcc/cp/parser.c:27060
0x865b4c cp_parser_template_declaration_after_export
        ../.././gcc/cp/parser.c:27079
0x865075 cp_parser_template_declaration_after_parameters
        ../.././gcc/cp/parser.c:26812
0x865b4c cp_parser_explicit_template_declaration
        ../.././gcc/cp/parser.c:27060
0x865b4c cp_parser_template_declaration_after_export
        ../.././gcc/cp/parser.c:27079
0x86bd79 cp_parser_declaration
        ../.././gcc/cp/parser.c:12788
0x86c0b1 cp_parser_declaration_seq_opt
        ../.././gcc/cp/parser.c:12715
0x86c3bf cp_parser_translation_unit
        ../.././gcc/cp/parser.c:4502
0x86c3bf c_parse_file()
        ../.././gcc/cp/parser.c:39021
0x9adff6 c_common_parse_file()
        ../.././gcc/c-family/c-opts.c:1124
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