ENH: add result_type_false: check the minimum promotion rules according to the spec

Conforming array libraries may extend the minimum promotion rules. If they do, they
will fail this new test and may want to xfail it.

Author: ev-br

Diff for: array_api_tests/hypothesis_helpers.py

@@ -10,7 +10,7 @@
 from hypothesis import assume, reject
 from hypothesis.strategies import (SearchStrategy, booleans, composite, floats,
                                    integers, just, lists, none, one_of,
-                                   sampled_from, shared, builds)
+                                   sampled_from, shared, builds, permutations)
 
 from . import _array_module as xp, api_version
 from . import array_helpers as ah
@@ -148,6 +148,25 @@ def mutually_promotable_dtypes(
     return one_of(strats).map(tuple)
 
 
+@composite
+def mutually_non_promotable_dtypes(
+    draw,
+    max_size: Optional[int] = 2,
+) -> Sequence[Tuple[DataType, ...]]:
+    """Generate a pair of dtypes which cannot be promoted."""
+    assert max_size == 2
+
+    _categories = [
+        (xp.bool,),
+        dh.uint_dtypes + dh.int_dtypes,
+        dh.real_float_dtypes + dh.complex_dtypes
+    ]
+    cat_st = permutations(_categories).map(lambda s: s[:2])
+    cat_from, cat_to = draw(cat_st)
+    from_, to = draw(sampled_from(cat_from)), draw(sampled_from(cat_to))
+    return from_, to
+
+
 class OnewayPromotableDtypes(NamedTuple):
     input_dtype: DataType
     result_dtype: DataType

Diff for: array_api_tests/test_data_type_functions.py

@@ -204,3 +204,13 @@ def test_isdtype(dtype, kind):
 def test_result_type(dtypes):
     out = xp.result_type(*dtypes)
     ph.assert_dtype("result_type", in_dtype=dtypes, out_dtype=out, repr_name="out")
+
+
+@given(hh.mutually_non_promotable_dtypes(2))
+def test_result_type_false(dtypes):
+    """Test _very_ strict promotion rules according to the spec.
+       Conforming array libraries may extend the promotion rules, and
+       then they'll need to xfail this test.
+    """
+    with pytest.raises(TypeError):
+        xp.result_type(*dtypes)