BUG: Fix numba DUFuncs added loops getting picked up

It was always my intention to do this: If no loop is found and we
go into the legacy ufunc path and legacy resolving works, we need
to double check that the ufunc was not mutated.

Normal operation never mutates ufuncs (its really not meant to be)
but numbas DUFuncs need to do it (they compile loops dynamically).
The future is much brighter for them in this regard, but right now
they have to keep working.

Closes gh-20735

numpy/core/src/umath/dispatching.c

@@ -746,6 +746,37 @@ promote_and_get_info_and_ufuncimpl(PyUFuncObject *ufunc,
     }
     info = promote_and_get_info_and_ufuncimpl(ufunc,
             ops, signature, new_op_dtypes, NPY_FALSE);
+    if (info == NULL) {
+        /*
+         * NOTE: This block exists solely to support numba's DUFuncs which add
+         * new loops dynamically, so our list may get outdated.  Thus, we
+         * have to make sure that the loop exists.
+         *
+         * Before adding a new loop, ensure that it actually exists. There
+         * is a tiny chance that this would not work, but it would require an
+         * extension additionally have a custom loop getter.
+         * This check should ensure a the right error message, but in principle
+         * we could try to call the loop getter here.
+         */
+        char *types = ufunc->types;
+        npy_bool loop_exists = NPY_FALSE;
+        for (int i = 0; i < ufunc->ntypes; ++i) {
+            loop_exists = NPY_TRUE;  /* assume it exists, break if not */
+            for (int j = 0; j < ufunc->nargs; ++j) {
+                if (types[j] != new_op_dtypes[j]->type_num) {
+                    loop_exists = NPY_FALSE;
+                    break;
+                }
+            }
+            types += ufunc->nargs;
+        }
+
+        if (loop_exists) {
+            info = add_and_return_legacy_wrapping_ufunc_loop(
+                    ufunc, new_op_dtypes, 0);
+        }
+    }
+
     for (int i = 0; i < ufunc->nargs; i++) {
         Py_XDECREF(new_op_dtypes[i]);
     }