Duplicate config defaults before setting

For scalar types used as config parameter default values, the existing logic is fine. But if a config default value is something like an array or hash, or any mutable object, we should not return the same instance every time the default value is set for that attribute on a config

Author: juniper-shopify

lib/roast/dsl/cog/config.rb

@@ -45,10 +45,12 @@ def [](key)
         class << self
           #: [T] (Symbol, T) ?{(T) -> T} -> void
           def field(key, default, &validator)
+            default = default #: as untyped
+
             define_method(key) do |*args|
               if args.empty?
                 # with no args, return the configured value, or the default
-                @values[key] || default
+                @values[key] || default.deep_dup
               else
                 # with an argument, set the configured value
                 new_value = args.first
@@ -58,7 +60,7 @@ def field(key, default, &validator)
 
             define_method("use_default_#{key}!".to_sym) do
               # explicitly set the configured value to the default
-              @values[key] = default
+              @values[key] = default.deep_dup
             end
           end
         end