[smart_holder] Remove obsolete detail::type_info::default_holder member. (#5541)

* git merge --squash purge_internals_versions_4_5

* Remove PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT, set PYBIND11_INTERNALS_VERSION 7

* Remove all uses of PYBIND11_SMART_HOLDER_ENABLED under include/pybind11

* Remove obsolete PYBIND11_ACTUALLY_USING_SMART_HOLDER_AS_DEFAULT macro.

* Remove PYBIND11_SMART_HOLDER_ENABLED in ubench/holder_comparison.cpp

* Remove all uses of PYBIND11_SMART_HOLDER_ENABLED under tests/

* Remove `#define PYBIND11_SMART_HOLDER_ENABLED`

* Remove all uses of PYBIND11_SMART_HOLDER_TYPE_CASTERS under tests/

* Remove all uses of PYBIND11_TYPE_CASTER_BASE_HOLDER under tests/

* Add missing `#include <cstdint>`

Example error message (🐍 3.11 • ubuntu-latest • x64, GNU 13.3.0):

```
include/pybind11/detail/value_and_holder.h:56:52: error: ‘uint8_t’ is not a member of ‘std’; did you mean ‘wint_t’?
   56 |             inst->nonsimple.status[index] &= (std::uint8_t) ~instance::status_holder_constructed;
      |                                                    ^~~~~~~
```

* Remove type_info::default_holder member. DOES NOT BUILD

* Remove some obsolete default_holder code and #ifdef out uses of typeinfo->default_holder. BUILDS BUT 2 TESTS ARE FAILING.

* Replace `default_holder` with `holder_enum_v == holder_enum_t::std_unique_ptr`

Intentionally not changing error messages, because this would result in a significantly bigger change.

* Change PYBIND11_INTERNALS_VERSION to 106: It will be changed to 7 in a follow-on PR that actually changes the internals.

* Change PYBIND11_INTERNALS_VERSION to 7 (because this PR actually changes the internals).

include/pybind11/attr.h

@@ -276,8 +276,7 @@ struct function_record {
 struct type_record {
     PYBIND11_NOINLINE type_record()
         : multiple_inheritance(false), dynamic_attr(false), buffer_protocol(false),
-          default_holder(true), module_local(false), is_final(false),
+          module_local(false), is_final(false), release_gil_before_calling_cpp_dtor(false) {}
-          release_gil_before_calling_cpp_dtor(false) {}
 
     /// Handle to the parent scope
     handle scope;
@@ -327,9 +326,6 @@ struct type_record {
     /// Does the class implement the buffer protocol?
     bool buffer_protocol : 1;
 
-    /// Is the default (unique_ptr) holder type used?
-    bool default_holder : 1;
-
     /// Is the class definition local to the module shared object?
     bool module_local : 1;
 
@@ -350,13 +346,17 @@ struct type_record {
                           + "\" referenced unknown base type \"" + tname + "\"");
         }
 
-        if (default_holder != base_info->default_holder) {
+        // SMART_HOLDER_BAKEIN_FOLLOW_ON: Refine holder compatibility checks.
+        bool this_has_unique_ptr_holder = (holder_enum_v == holder_enum_t::std_unique_ptr);
+        bool base_has_unique_ptr_holder
+            = (base_info->holder_enum_v == holder_enum_t::std_unique_ptr);
+        if (this_has_unique_ptr_holder != base_has_unique_ptr_holder) {
             std::string tname(base.name());
             detail::clean_type_id(tname);
             pybind11_fail("generic_type: type \"" + std::string(name) + "\" "
-                          + (default_holder ? "does not have" : "has")
+                          + (this_has_unique_ptr_holder ? "does not have" : "has")
                           + " a non-default holder type while its base \"" + tname + "\" "
-                          + (base_info->default_holder ? "does not" : "does"));
+                          + (base_has_unique_ptr_holder ? "does not" : "does"));
         }
 
         bases.append((PyObject *) base_info->type);

include/pybind11/cast.h

@@ -790,7 +790,10 @@ public:
 protected:
     friend class type_caster_generic;
     void check_holder_compat() {
-        if (typeinfo->default_holder) {
+        // SMART_HOLDER_BAKEIN_FOLLOW_ON: Refine holder compatibility checks.
+        bool inst_has_unique_ptr_holder
+            = (typeinfo->holder_enum_v == holder_enum_t::std_unique_ptr);
+        if (inst_has_unique_ptr_holder) {
             throw cast_error("Unable to load a custom holder type from a default-holder instance");
         }
     }
@@ -908,7 +911,10 @@ public:
 protected:
     friend class type_caster_generic;
     void check_holder_compat() {
-        if (typeinfo->default_holder) {
+        // SMART_HOLDER_BAKEIN_FOLLOW_ON: Refine holder compatibility checks.
+        bool inst_has_unique_ptr_holder
+            = (typeinfo->holder_enum_v == holder_enum_t::std_unique_ptr);
+        if (inst_has_unique_ptr_holder) {
             throw cast_error("Unable to load a custom holder type from a default-holder instance");
         }
     }

include/pybind11/detail/internals.h

@@ -37,11 +37,11 @@
 /// further ABI-incompatible changes may be made before the ABI is officially
 /// changed to the new version.
 #ifndef PYBIND11_INTERNALS_VERSION
-#    define PYBIND11_INTERNALS_VERSION 106
+#    define PYBIND11_INTERNALS_VERSION 7
 #endif
 
-#if PYBIND11_INTERNALS_VERSION < 106
+#if PYBIND11_INTERNALS_VERSION < 7
-#    error "PYBIND11_INTERNALS_VERSION 106 is the minimum (SPECIAL SITUATION)."
+#    error "PYBIND11_INTERNALS_VERSION 7 is the minimum for all platforms for pybind11v3."
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
@@ -234,6 +234,7 @@ struct type_info {
     buffer_info *(*get_buffer)(PyObject *, void *) = nullptr;
     void *get_buffer_data = nullptr;
     void *(*module_local_load)(PyObject *, const type_info *) = nullptr;
+    holder_enum_t holder_enum_v = holder_enum_t::undefined;
     /* A simple type never occurs as a (direct or indirect) parent
      * of a class that makes use of multiple inheritance.
      * A type can be simple even if it has non-simple ancestors as long as it has no descendants.
@@ -241,13 +242,8 @@ struct type_info {
     bool simple_type : 1;
     /* True if there is no multiple inheritance in this type's inheritance tree */
     bool simple_ancestors : 1;
-    /* for base vs derived holder_type checks */
-    // SMART_HOLDER_BAKEIN_FOLLOW_ON: Remove default_holder member here and
-    // produce better error messages in the places where it is currently used.
-    bool default_holder : 1;
     /* true if this is a type registered with py::module_local */
     bool module_local : 1;
-    holder_enum_t holder_enum_v = holder_enum_t::undefined;
 };
 
 #define PYBIND11_INTERNALS_ID                                                                     \

include/pybind11/pybind11.h

@@ -1446,7 +1446,6 @@ protected:
         tinfo->dealloc = rec.dealloc;
         tinfo->simple_type = true;
         tinfo->simple_ancestors = true;
-        tinfo->default_holder = rec.default_holder;
         tinfo->module_local = rec.module_local;
         tinfo->holder_enum_v = rec.holder_enum_v;
 
@@ -1903,9 +1902,6 @@ public:
         record.holder_size = sizeof(holder_type);
         record.init_instance = init_instance;
 
-        // A more fitting name would be uses_unique_ptr_holder.
-        record.default_holder = detail::is_instantiation<std::unique_ptr, holder_type>::value;
-
         if (detail::is_instantiation<std::unique_ptr, holder_type>::value) {
             record.holder_enum_v = detail::holder_enum_t::std_unique_ptr;
         } else if (detail::is_instantiation<std::shared_ptr, holder_type>::value) {