Track base class pointers of instances

This commits adds base class pointers of offset base classes (i.e. due
to multiple inheritance) to `registered_instances` so that if such a
pointer is returned we properly recognize it as an existing instance.

Without this, returning a base class pointer will cast to the existing
instance if the pointer happens to coincide with the instance pointer,
but constructs a new instance (quite possibly with a segfault, if
ownership is applied) for unequal base class pointers due to multiple
inheritance.

tests/test_multiple_inheritance.py

@@ -112,6 +112,32 @@ def test_mi_dynamic_attributes():
         assert d.dynamic == 1
 
 
+def test_mi_unaligned_base():
+    """Returning an offset (non-first MI) base class pointer should recognize the instance"""
+    from pybind11_tests import I801C, I801D, i801b1_c, i801b2_c, i801b1_d, i801b2_d
+
+    n_inst = ConstructorStats.detail_reg_inst()
+
+    c = I801C()
+    d = I801D()
+    # + 4 below because we have the two instances, and each instance has offset base I801B2
+    assert ConstructorStats.detail_reg_inst() == n_inst + 4
+    b1c = i801b1_c(c)
+    assert b1c is c
+    b2c = i801b2_c(c)
+    assert b2c is c
+    b1d = i801b1_d(d)
+    assert b1d is d
+    b2d = i801b2_d(d)
+    assert b2d is d
+
+    assert ConstructorStats.detail_reg_inst() == n_inst + 4  # no extra instances
+    del c, b1c, b2c
+    assert ConstructorStats.detail_reg_inst() == n_inst + 2
+    del d, b1d, b2d
+    assert ConstructorStats.detail_reg_inst() == n_inst
+
+
 def test_mi_base_return():
     """Tests returning an offset (non-first MI) base class pointer to a derived instance"""
     from pybind11_tests import (I801B2, I801C, I801D, i801c_b1, i801c_b2, i801d_b1, i801d_b2,
@@ -129,7 +155,7 @@ def test_mi_base_return():
     assert d1.a == 1
     assert d1.b == 2
 
-    assert ConstructorStats.detail_reg_inst() == n_inst + 2
+    assert ConstructorStats.detail_reg_inst() == n_inst + 4
 
     c2 = i801c_b2()
     assert type(c2) is I801C
@@ -141,10 +167,10 @@ def test_mi_base_return():
     assert d2.a == 1
     assert d2.b == 2
 
-    assert ConstructorStats.detail_reg_inst() == n_inst + 4
+    assert ConstructorStats.detail_reg_inst() == n_inst + 8
 
     del c2
-    assert ConstructorStats.detail_reg_inst() == n_inst + 3
+    assert ConstructorStats.detail_reg_inst() == n_inst + 6
     del c1, d1, d2
     assert ConstructorStats.detail_reg_inst() == n_inst