[Bug c++/121330] New: g++ hangs after compilation error in deeply nested template

[jirehguo at tju dot edu.cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121330

            Bug ID: 121330
           Summary: g++ hangs after compilation error in deeply nested
                    template
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jirehguo at tju dot edu.cn
  Target Milestone: ---

The following code causes both Clang and GCC to produce an error due to the
undeclared identifier 'func'. However, there is a critical difference in
behavior:

Clang: Prints the error and exits immediately.
GCC: Prints the error but then hangs, failing to terminate.
Repro: https://godbolt.org/z/dfEf55318

Code:

template <typename... Ts> struct Foo {
  template <typename... T> static void ignore() {
    func();  
  }
  Foo() { ignore<Ts...>(); }
};

struct Base {
  Base();
  ~Base();
};

#define STAMP(thiz, prev)                                                     
\
  using thiz = Foo<prev, prev, prev, prev, prev, prev, prev, prev, prev, prev,
\
                   prev, prev, prev, prev, prev, prev, prev, prev, prev, prev,
\
                   prev, prev, prev, prev, prev, prev, prev, prev, prev>;
STAMP(A, Base);
STAMP(B, A);
STAMP(C, B);
STAMP(D, C);
STAMP(E, D);
STAMP(F, E);
STAMP(G, F);

int main() {
  G g;
}