with_container_arithmetic: Nix implementation of __eq__ if eq_comparison not requested

arraycontext/container/arithmetic.py

@@ -304,12 +304,25 @@ def with_container_arithmetic(
         # {{{ binary operators
 
         for dunder_name, op_str, reversible, op_cls in _BINARY_OP_AND_DUNDER:
+            fname = f"_{cls.__name__.lower()}_{dunder_name}"
+
             if op_cls not in desired_op_classes:
+                # Leaving equality comparison at the default supplied by
+                # dataclasses is dangerous: Comparison of dataclass fields
+                # might return an array of truth values, and the dataclasses
+                # implementation of __eq__ might consider that 'truthy' enough,
+                # yielding bogus equality results.
+                if op_cls == _OpClass.EQ_COMPARISON:
+                    gen(f"def {fname}(arg1, arg2):")
+                    with Indentation(gen):
+                        gen("return NotImplemented")
+                    gen(f"cls.__{dunder_name}__ = {fname}")
+                    gen("")
+
                 continue
 
             # {{{ "forward" binary operators
 
-            fname = f"_{cls.__name__.lower()}_{dunder_name}"
             zip_init_args = cls._deserialize_init_arrays_code("arg1", {
                     same_key(key_arg1, key_arg2):
                     _format_binary_op_str(op_str, expr_arg1, expr_arg2)