Fix build with GCC 15 and runtime issue with libstdc++ 15.1.
https://github.com/CleverRaven/Cataclysm-DDA/pull/80800

From ac41a58401b663e8ae27d7352eb95795bf778dac Mon Sep 17 00:00:00 2001
From: avaliente-bc <56400759+avaliente-bc@users.noreply.github.com>
Date: Wed, 6 Apr 2022 21:27:37 +0200
Subject: [PATCH 1/2] stl_emulation span::count_ is not const anymore

In C++ we cannot have both assignment operator and const member. Since
span::operator= is defined, span::count_ constness must be removed.

cherry-picked from upstream
google/flatbuffers@20aad0c41e1252b04c72111c3eb221280a9c2009.

fixes:

	In file included from ../src/third-party/flatbuffers/util.h:25,
			 from ../src/third-party/flatbuffers/flexbuffers.h:26,
			 from ../src/flexbuffer_json.h:9,
			 from ../src/json.h:1623,
			 from ../src/units.h:17,
			 from ../src/damage.h:18,
			 from ../src/bodypart.h:15,
			 from ../src/avatar.h:16,
			 from ../tests/battery_mod_test.cpp:9:
	../src/third-party/flatbuffers/stl_emulation.h: In member function ‘constexpr flatbuffers::span<T, Extent>& flatbuffers::span<T, Extent>::operator=(const flatbuffers::span<T, Extent>&)’:
	../src/third-party/flatbuffers/stl_emulation.h:550:12: error: assignment of read-only member ‘flatbuffers::span<T, Extent>::count_’
	  550 |     count_ = other.count_;
	      |     ~~~~~~~^~~~~~~~~~~~~~

under gcc 14.
--- a/src/third-party/flatbuffers/stl_emulation.h
+++ b/src/third-party/flatbuffers/stl_emulation.h
@@ -626,7 +626,7 @@ class span FLATBUFFERS_FINAL_CLASS {
  private:
   // This is a naive implementation with 'count_' member even if (Extent != dynamic_extent).
   pointer const data_;
-  const size_type count_;
+  size_type count_;
 };
 
  #if !defined(FLATBUFFERS_SPAN_MINIMAL)

From 531de9110868acf2df825bce0368de4fbbcb8a13 Mon Sep 17 00:00:00 2001
From: andrei <a12l+git@runbox.com>
Date: Fri, 2 May 2025 23:23:27 +0300
Subject: [PATCH 2/2] mapdata: ensure EMPTY_GROUP is initialized before
 generic_factory<field_type>

--- a/src/item_factory.cpp
+++ b/src/item_factory.cpp
@@ -87,8 +87,6 @@ static const item_category_id item_category_tools( "tools" );
 static const item_category_id item_category_veh_parts( "veh_parts" );
 static const item_category_id item_category_weapons( "weapons" );
 
-static const item_group_id Item_spawn_data_EMPTY_GROUP( "EMPTY_GROUP" );
-
 static const material_id material_bean( "bean" );
 static const material_id material_blood( "blood" );
 static const material_id material_bone( "bone" );
@@ -1891,7 +1889,7 @@ void Item_factory::init()
     add_actor( std::make_unique<effect_on_conditons_actor>() );
     // An empty dummy group, it will not spawn anything. However, it makes that item group
     // id valid, so it can be used all over the place without need to explicitly check for it.
-    m_template_groups[Item_spawn_data_EMPTY_GROUP] =
+    m_template_groups[get_Item_spawn_data_EMPTY_GROUP()] =
         std::make_unique<Item_group>( Item_group::G_COLLECTION, 100, 0, 0, "EMPTY_GROUP" );
 }
 
--- a/src/item_group.cpp
+++ b/src/item_group.cpp
@@ -67,6 +67,12 @@ void Item_spawn_data::relic_generator::load( const JsonObject &jo )
     mandatory( jo, was_loaded, "procgen_id", id );
 }
 
+item_group_id get_Item_spawn_data_EMPTY_GROUP()
+{
+    static const item_group_id Item_spawn_data_EMPTY_GROUP( "EMPTY_GROUP" );
+    return Item_spawn_data_EMPTY_GROUP;
+}
+
 relic Item_spawn_data::relic_generator::generate_relic( const itype_id &it_id ) const
 {
     return id->generate( rules, it_id );
--- a/src/item_group.h
+++ b/src/item_group.h
@@ -411,4 +411,6 @@ class Item_group : public Item_spawn_data
         prop_list items;
 };
 
+item_group_id get_Item_spawn_data_EMPTY_GROUP();
+
 #endif // CATA_SRC_ITEM_GROUP_H
--- a/src/mapdata.cpp
+++ b/src/mapdata.cpp
@@ -26,8 +26,6 @@
 #include "trap.h"
 #include "type_id.h"
 
-static const item_group_id Item_spawn_data_EMPTY_GROUP( "EMPTY_GROUP" );
-
 namespace
 {
 
@@ -340,7 +338,7 @@ map_bash_info::map_bash_info() : str_min( -1 ), str_max( -1 ),
     str_min_supported( -1 ), str_max_supported( -1 ),
     explosive( 0 ), sound_vol( -1 ), sound_fail_vol( -1 ),
     collapse_radius( 1 ), destroy_only( false ), bash_below( false ),
-    drop_group( Item_spawn_data_EMPTY_GROUP ),
+    drop_group( get_Item_spawn_data_EMPTY_GROUP() ),
     ter_set( ter_str_id::NULL_ID() ), furn_set( furn_str_id::NULL_ID() ) {}
 
 bool map_bash_info::load( const JsonObject &jsobj, const std::string_view member,
@@ -395,7 +393,7 @@ bool map_bash_info::load( const JsonObject &jsobj, const std::string_view member
         drop_group = item_group::load_item_group( j.get_member( "items" ), "collection",
                      "map_bash_info for " + context );
     } else {
-        drop_group = Item_spawn_data_EMPTY_GROUP;
+        drop_group = get_Item_spawn_data_EMPTY_GROUP();
     }
 
     if( j.has_array( "tent_centers" ) ) {
