alias used on a expression in the same projection should fail always

[ajnavarro]

the following query:

select 
    language(blob_content, file_path) as lang, 
    substring(lang,1,1) as result
from refs 
NATURAL JOIN commits 
NATURAL JOIN commit_files 
NATURAL JOIN files 
where lang != '' limit 100;

Is returning results, but it should fail with the following error:
ERROR 1105 (HY000): unknown error: column "lang" does not exist in scope, but there is an alias defined in this projection with that name. Aliases cannot be used in the same projection they're defined in