DEV: Raise an exception if trying to set a readonly column with default (#27416)

lib/migration/column_dropper.rb

@@ -5,6 +5,15 @@ require "migration/base_dropper"
 module Migration
   class ColumnDropper
     def self.mark_readonly(table_name, column_name)
+      has_default = DB.query_single(<<~SQL, table_name: table_name, column_name: column_name).first
+        SELECT column_default IS NOT NULL
+        FROM information_schema.columns
+        WHERE table_name = :table_name
+        AND column_name = :column_name
+      SQL
+
+      raise "You must drop a column's default value before marking it as readonly" if has_default
+
       BaseDropper.create_readonly_function(table_name, column_name)
 
       DB.exec <<~SQL