Currently, fourmolu formats type application with ticked promoted constructor with space inserted in-between:

before:

foo @'True

after:

foo @ 'True

This works fine with GHC <= 8.10.

However, as GHC 9.0 introduces whitespace-sensitive parsing of operators (1, 2), the above result makes ghc complaining that "Variable not in scope: (@)".

Dependently-typed programs suffer much from this behaviour, and hence a whitespace MUST NOT be inserted.