[Bug c++/126752] New: ICE in build_vec_init when initializing an array member of an aggregate

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126752

            Bug ID: 126752
           Summary: ICE in build_vec_init when initializing an array
                    member of an aggregate
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: s.kimura.h41104 at gmail dot com
  Target Milestone: ---

Compiler Explorer: https://godbolt.org/z/MhTo7e9sf

Reproducer:
```
struct C {
  struct V_impl {
    constexpr V_impl() {}
  };
  struct V_base {
    ~V_base();
    V_impl v_impl;
  };
  struct vec : private V_base {};
  struct S {
    S();
  };
  struct V {
    vec vv{};
  };
  struct FOO {
    V v[1]{};
    S s;
  };
  void foo() { FOO{}; }
};
```

Backtrace:
<source>: In member function 'void C::foo()':
<source>:20:20: internal compiler error: in build_vec_init, at cp/init.cc:5368
   20 |   void foo() { FOO{}; }
      |                    ^
0x2b97f28 diagnostics::context::diagnostic_impl(rich_location*,
diagnostics::metadata const*, diagnostics::option_id, char const*,
__va_list_tag (*) [1], diagnostics::kind)
        ???:0
0x2b8cb6b internal_error(char const*, ...)
        ???:0
0xb40b7e fancy_abort(char const*, int, char const*)
        ???:0
0xe65584 expand_vec_init_expr(tree_node*, tree_node*, int, vec<tree_node*,
va_gc, vl_embed>**)
        ???:0
0xea3bb7 split_nonconstant_init(tree_node*, tree_node*)
        ???:0
0x1949d73 walk_tree_1(tree_node**, tree_node* (*)(tree_node**, int*, void*),
void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*,
tree_node* (*)(tree_node**, int*, tree_node* (*)(tree_node**, int*, void*),
void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*))
        ???:0
0xbf1b33 cp_fold_function(tree_node*)
        ???:0
0xc48837 finish_function(bool)
        ???:0
0xd96823 c_parse_file()
        ???:0
0xf29cf9 c_common_parse_file()
        ???:0

This ICE goes back to gcc-16:
https://godbolt.org/z/ffKno4Y4n