The warning was (controversially) added so that people don't inadvertently use prefix notation when infix was wanted like 1 ==2.
1 ==2 is parsed as (1, (==`(2)), i.e. `== is prefix while 1 == 2 is parsed as (==, 1, 2) so == is infix. This is especially important for the operator with both prefix and infix forms like - and &. By mistake you could use: a &b instead of a & b. More details in [#7685](https://github.com/nim-lang/Nim/issues/7685).
