"Such a discussion" is correct. "Such discussion" is ungrammatical. When in doubt about whether or not to include an article (definite or indefinite), always put the article in.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Anonymous"Such discussion" is ungrammatical.It is perfectly grammatical if you use 'discussion' uncountably.
Anonymousalways put articles in when in doubt.Only if you are sure that you are dealing with countable nouns.