Add tests for generic bool conversions

tests/test_numpy_dtypes.cpp

@@ -467,7 +467,8 @@ test_initializer numpy_dtypes([](py::module &m) {
     m.def("f_packed", [](PackedStruct s) { return s.uint_ * 10; });
     m.def("f_nested", [](NestedStruct s) { return s.a.uint_ * 10; });
     m.def("register_dtype", []() { PYBIND11_NUMPY_DTYPE(SimpleStruct, bool_, uint_, float_, ldbl_); });
-    m.def("negate_bool", [](bool arg) { return !arg; });
+    m.def("bool_passthrough", [](bool arg) { return arg; });
+    m.def("bool_passthrough_noconvert", [](bool arg) { return arg; }, py::arg().noconvert());
 });
 
 #undef PYBIND11_PACKED

tests/test_numpy_dtypes.py

@@ -323,6 +323,51 @@ def test_compare_buffer_info():
 
 @pytest.requires_numpy
 def test_numpy_bool():
-    from pybind11_tests import negate_bool
-    assert negate_bool(np.bool_(True)) is False
-    assert negate_bool(np.bool_(False)) is True
+    from pybind11_tests import bool_passthrough as convert, bool_passthrough_noconvert as noconvert
+
+    require_implicit = lambda v: pytest.raises(TypeError, noconvert, v)
+    cant_convert = lambda v: pytest.raises(TypeError, convert, v)
+
+    # straight up bool
+    assert convert(True) is True
+    assert convert(False) is False
+    assert noconvert(True) is True
+    assert noconvert(False) is False
+
+    # np.bool_ is not considered implicit
+    assert convert(np.bool_(True)) is True
+    assert convert(np.bool_(False)) is False
+    assert noconvert(np.bool_(True)) is True
+    assert noconvert(np.bool_(False)) is False
+
+    # None requires implicit conversion
+    require_implicit(None)
+    assert convert(None) is False
+
+    # Sequence types check for lengths (same as in PyObject_IsTrue)
+    require_implicit([])
+    require_implicit(())
+    require_implicit('')
+    assert convert([]) is False
+    assert convert([1]) is True
+    assert convert(()) is False
+    assert convert((0,)) is True
+    assert convert('') is False
+    assert convert('foo') is True
+
+    class A(object):
+        def __init__(self, x): self.x = x
+        def __nonzero__(self): return self.x
+        __bool__ = __nonzero__
+
+    class B(object):
+        pass
+
+    # Arbitrary objects are not accepted
+    cant_convert(object())
+    cant_convert(B())
+
+    # Objects with __nonzero__ / __bool__ defined can be converted
+    require_implicit(A(True))
+    assert convert(A(True)) is True
+    assert convert(A(False)) is False