[Bug target/114194] New: ICE when using std::unique_ptr with xtheadvector

["camel-cdr at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114194

            Bug ID: 114194
           Summary: ICE when using std::unique_ptr with xtheadvector
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: camel-cdr at protonmail dot com
  Target Milestone: ---

Using std::unique_ptr with xtheadvector enabled causes an ICE:

#include <memory>
extern void use(std::unique_ptr<int> &x);
void test(size_t n) {
        std::unique_ptr<int> x;
        use(x);
}

See also: https://godbolt.org/z/6nbhxKdfd

I've managed to reduce the problem to the following set of templates, but I
have no idea how this could cause the ICE.

struct S1 { int x; };
struct S2 { constexpr S2() { } template<class T> S2(T&); };
struct S3 : S1, S2 { constexpr S3() : S1() { } S3(S3&); };
void f(S3 &) { S3 x; f(x); }


See also: https://godbolt.org/z/5YxM6jd3s

It's extremely brittle, the ICE goes away, if you remove constexpr, the
reference, or any other part I could think of.