Adding a few comments after review by @laramiel.

include/pybind11/detail/init.h

@@ -182,8 +182,13 @@ void construct(value_and_holder &v_h, std::unique_ptr<Cpp<Class>, D> &&unq_ptr,
     if (Class::has_alias && need_alias && !is_alias<Class>(ptr))
         throw type_error("pybind11::init(): construction failed: returned std::unique_ptr pointee "
                          "is not an alias instance");
+    // Here and below: if the new object is a trampoline, the shared_from_this mechanism needs
+    // to be prevented from accessing the smart_holder vptr, because it does not keep the
+    // trampoline Python object alive. For types that don't inherit from enable_shared_from_this
+    // it does not matter if void_cast_raw_ptr is true or false, therefore it's not necessary
+    // to also inspect the type.
     auto smhldr = type_caster<Cpp<Class>>::template smart_holder_from_unique_ptr(
-        std::move(unq_ptr), Class::has_alias && is_alias<Class>(ptr));
+        std::move(unq_ptr), /*void_cast_raw_ptr*/ Class::has_alias && is_alias<Class>(ptr));
     v_h.value_ptr() = ptr;
     v_h.type->init_instance(v_h.inst, &smhldr);
 }
@@ -198,7 +203,7 @@ void construct(value_and_holder &v_h,
     auto *ptr = unq_ptr.get();
     no_nullptr(ptr);
     auto smhldr = type_caster<Alias<Class>>::template smart_holder_from_unique_ptr(
-        std::move(unq_ptr), true);
+        std::move(unq_ptr), /*void_cast_raw_ptr*/ true);
     v_h.value_ptr() = ptr;
     v_h.type->init_instance(v_h.inst, &smhldr);
 }

tests/test_class_sh_trampoline_shared_from_this.py

@@ -9,6 +9,8 @@ class PySft(m.Sft):
 
 
 def test_release_and_shared_from_this():
+    # Exercises the most direct path from building a shared_from_this-visible
+    # shared_ptr to calling shared_from_this.
     obj = PySft("PySft")
     assert obj.history == "PySft"
     assert m.use_count(obj) == 1
@@ -30,6 +32,7 @@ def test_release_and_shared_from_this_leak():
 
 
 def test_release_and_stash():
+    # Exercises correct functioning of guarded_delete weak_ptr.
     obj = PySft("PySft")
     stash1 = m.SftSharedPtrStash(1)
     stash1.Add(obj)
@@ -88,6 +91,7 @@ def test_release_and_stash_leak():
 
 
 def test_release_and_stash_via_shared_from_this():
+    # Exercises that the smart_holder vptr is invisible to the shared_from_this mechnism.
     obj = PySft("PySft")
     stash1 = m.SftSharedPtrStash(1)
     with pytest.raises(RuntimeError) as exc_info:
@@ -120,6 +124,8 @@ def test_release_and_stash_via_shared_from_this_leak():
 
 
 def test_pass_released_shared_ptr_as_unique_ptr():
+    # Exercises that returning a unique_ptr fails while a shared_from_this
+    # visible shared_ptr exists.
     obj = PySft("PySft")
     stash1 = m.SftSharedPtrStash(1)
     stash1.Add(obj)  # Releases shared_ptr to C++.
@@ -139,6 +145,7 @@ def test_pass_released_shared_ptr_as_unique_ptr():
     ],
 )
 def test_pure_cpp_sft_raw_ptr(make_f):
+    # Exercises void_cast_raw_ptr logic for different situations.
     obj = make_f("PureCppSft")
     assert m.pass_shared_ptr(obj) == 3
     assert obj.history == "PureCppSft_PassSharedPtr"