Poita/d-programming-language.org — comment on commit c3fa57dc843aadb8607a4d87c4a9248a76738ac2
File: lex.dd line 569
By Poita:

That's the point, it's not an error. @ is a valid token therefore q{ @ } is not an error. Also, dmd accepts it without error.

See: http://www.d-programming-language.org/lex.html

@ is clearly listed in the Tokens section.