[Bug c++/101247] New: ICE when using static constexpr bool defined in base-class in derived class constructor requires-clause

["lewissbaker.opensource at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101247

            Bug ID: 101247
           Summary: ICE when using static constexpr bool defined in
                    base-class in derived class constructor
                    requires-clause
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lewissbaker.opensource at gmail dot com
  Target Milestone: ---

Compiling the following code with GCC trunk and flags: -std=c++20

```
template<typename T>
struct in_place_type_t {};

template<typename T>
inline constexpr in_place_type_t<T> in_place_type;

template<typename T, typename... Ts>
inline constexpr bool outer_helper_v = true;

template<typename... Ts>
struct foo {
    struct type;

    template<typename U>
    static constexpr bool helper_v = outer_helper_v<U, Ts...>;
};

template<typename... Ts>
struct foo<Ts...>::type {
    template<typename U>
        requires helper_v<U>
    type(in_place_type_t<U>) {}
};

int main() {
    foo<int>::type x(in_place_type<int>);
}
```

results in an internal compiler-error

```
<source>: In substitution of 'template<class U>  requires  helper_v<U>
foo<int>::type::type(in_place_type_t<T>) [with U = {int}]':
<source>:26:40:   required from here
<source>:21:18: internal compiler error: tree check: accessed elt 1 of
'tree_vec' with 0 elts in tsubst_pack_expansion, at cp/pt.c:13084
   21 |         requires helper_v<U>
      |                  ^~~~~~~~~~~
0x1d4c179 internal_error(char const*, ...)
        ???:0
0x685746 tree_vec_elt_check_failed(int, int, char const*, int, char const*)
        ???:0
0x99e0bc tsubst_pack_expansion(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x9afa62 tsubst_template_args(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x9b064b tsubst_argument_pack(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x9afa04 tsubst_template_args(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x983ebd tsubst(tree_node*, tree_node*, int, tree_node*)
        ???:0
0x7abcda constraints_satisfied_p(tree_node*, tree_node*)
        ???:0
0x9c26c3 fn_type_unification(tree_node*, tree_node*, tree_node*, tree_node*
const*, unsigned int, tree_node*, unification_kind_t, int, conversion**, bool,
bool)
        ???:0
0x74abc9 build_new_method_call(tree_node*, tree_node*, vec<tree_node*, va_gc,
vl_embed>**, tree_node*, int, tree_node**, int)
        ???:0
0x74c8c0 build_special_member_call(tree_node*, tree_node*, vec<tree_node*,
va_gc, vl_embed>**, tree_node*, int, int)
        ???:0
0x8611c8 build_aggr_init(tree_node*, tree_node*, int, int)
        ???:0
0x81f0d9 cp_finish_decl(tree_node*, tree_node*, bool, tree_node*, int)
        ???:0
0x94f36d c_parse_file()
        ???:0
0xad2922 c_common_parse_file()
        ???:0
```

See https://godbolt.org/z/ebnxfsMs9 for an example.