More realistic example extracted from gcc's poly_int:

// $ cat rtl-tests.cc
template<unsigned int N> struct poly_int {
  template<typename ...Cs> constexpr poly_int (const Cs &... cs)
  : coeffs { cs... } {}

  int coeffs[N];
};

#define TARGET_DEFINED_VALUE 1
// this works:
//#define TARGET_DEFINED_VALUE 2

// Is instantiated only for N == 2.
template<unsigned int N> struct const_poly_int_tests {
  static void run () {
    poly_int<TARGET_DEFINED_VALUE> (1, 1);
  }
};

$ g++ -c rtl-tests.cc -fchecking=1
# did not fail, BAD!

$ g++ -c rtl-tests.cc -fchecking=2
rtl-tests.cc: In instantiation of 'constexpr poly_int<N>::poly_int(const Cs& ...) [with Cs = {int, int}; unsigned int N = 1]':
rtl-tests.cc:15:42:   required from here
rtl-tests.cc:3:5: error: too many initializers for 'int [1]'
    3 |   : coeffs { cs... } {}
      |     ^~~~~~~~~~~~~~~~
# failed, GOOD

$ clang++ -c rtl-tests.cc
rtl-tests.cc:3:14: error: excess elements in array initializer
  : coeffs { cs... } {}
             ^~
rtl-tests.cc:15:5: note: in instantiation of function template specialization 'poly_int<1>::poly_int<int, int>' requested here
    poly_int<TARGET_DEFINED_VALUE> (1, 1);
    ^
1 error generated.
# failed, GOOD

We probably want to diagnose this regardless of -fchecking.

Here's a similar IFNDR example that we don't diagnose even with -fchecking=2.

struct A { };

void g(A);

template<class T>
void f() {
  g(A(42));
}