[Bug c++/116888] New: Incorrect lazy substitution into unevaluated context

[mizvekov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116888

            Bug ID: 116888
           Summary: Incorrect lazy substitution into unevaluated context
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mizvekov at gmail dot com
  Target Milestone: ---

GCC misses diagnosing this invalid call when the outer template is
instantiated:

https://godbolt.org/z/jxTWY9x6z

  template <class T> void d() {}

  template <class T> struct Test1 {
    template <class U, class = void> struct Helper {};
    template <class U> struct Helper<U, decltype(d<T>(1))>;
  };

  template struct Test1<int>;

GCC accepts this, while every other implementation rejects.