Allow migrating from deleted variants

On loading a variant that is not a possible variant from the given item,
try to search from a migration from that item with the variant that was
loaded, and apply that migration if relevant.

src/item_factory.cpp

@@ -4563,35 +4563,8 @@ itype_id Item_factory::migrate_id( const itype_id &id )
     return parent != nullptr ? parent->replace : id;
 }
 
-void Item_factory::migrate_item( const itype_id &id, item &obj )
+static void apply_migration( const migration *migrant, item &obj )
 {
-    auto iter = migrations.find( id );
-    if( iter == migrations.end() ) {
-        return;
-    }
-    bool convert = false;
-    const migration *migrant = nullptr;
-    for( const migration &m : iter->second ) {
-        if( m.from_variant && obj.has_itype_variant() && obj.itype_variant().id == *m.from_variant ) {
-            migrant = &m;
-            // This is not the variant that the item has already been convert to
-            // So we'll convert it again.
-            convert = true;
-            break;
-        }
-        // When we find a migration that doesn't care about variants, keep it around
-        if( !m.from_variant ) {
-            migrant = &m;
-        }
-    }
-    if( migrant == nullptr ) {
-        return;
-    }
-
-    if( convert ) {
-        obj.convert( migrant->replace );
-    }
-
     if( migrant->reset_item_vars ) {
         obj.clear_vars();
         for( const auto &pair : migrant->replace.obj().item_variables ) {
@@ -4629,6 +4602,54 @@ void Item_factory::migrate_item( const itype_id &id, item &obj )
     }
 }
 
+void Item_factory::migrate_item( const itype_id &id, item &obj )
+{
+    auto iter = migrations.find( id );
+    if( iter == migrations.end() ) {
+        return;
+    }
+    bool convert = false;
+    const migration *migrant = nullptr;
+    for( const migration &m : iter->second ) {
+        if( m.from_variant && obj.has_itype_variant() && obj.itype_variant().id == *m.from_variant ) {
+            migrant = &m;
+            // This is not the variant that the item has already been convert to
+            // So we'll convert it again.
+            convert = true;
+            break;
+        }
+        // When we find a migration that doesn't care about variants, keep it around
+        if( !m.from_variant ) {
+            migrant = &m;
+        }
+    }
+    if( migrant == nullptr ) {
+        return;
+    }
+
+    if( convert ) {
+        obj.convert( migrant->replace );
+    }
+
+    apply_migration( migrant, obj );
+}
+
+void Item_factory::migrate_item_from_variant( item &obj, const std::string &from_variant )
+{
+    auto iter = migrations.find( obj.typeId() );
+    if( iter == migrations.end() ) {
+        return;
+    }
+    for( const migration &m : iter->second ) {
+        if( !m.from_variant.has_value() || m.from_variant.value() != from_variant ) {
+            continue;
+        }
+        obj.convert( m.replace );
+        apply_migration( &m, obj );
+        break;
+    }
+}
+
 void Item_factory::set_qualities_from_json( const JsonObject &jo, const std::string &member,
         itype &def )
 {

src/item_factory.h

@@ -224,6 +224,9 @@ class Item_factory
          */
         void migrate_item( const itype_id &id, item &obj );
 
+        /** applies a migration to the item if one exists with the given from_variant */
+        void migrate_item_from_variant( item &obj, const std::string &from_variant );
+
         /**
          * Check if an item type is known to the Item_factory.
          * @param id Item type id (@ref itype::id).

src/savegame_json.cpp

@@ -2875,7 +2875,11 @@ void item::io( Archive &archive )
     } );
     archive.io( "craft_data", craft_data_, decltype( craft_data_ )() );
     const auto ivload = [this]( const std::string & variant ) {
-        set_itype_variant( variant );
+        if( possible_itype_variant( variant ) ) {
+            set_itype_variant( variant );
+        } else {
+            item_controller->migrate_item_from_variant( *this, variant );
+        }
     };
     const auto ivsave = []( const itype_variant_data * iv ) {
         return iv->id;