[language] error on undefined key in struct and table

anselme/ast/Struct.lua

@@ -113,7 +113,7 @@ Struct = ast.abstract.Runtime {
 		if v ~= nil then
 			return v
 		else
-			return Nil:new()
+			error(("key %q is undefined in %s"):format(key:format(), self.type), 0)
 		end
 	end,
 	get_strict = function(self, key)

ideas.md

@@ -25,10 +25,6 @@ Issue: dispatch is decided before evaluating default values.
 
 Multiline string and comments?
 
----
-
-Error on undefined struct/table key? I copied the Lua behavior but maybe not useful here.
-
 # Can be done later
 
 Translation.