Fix the treatment of 'closed' definitions
The IdBindingInfo field of ATcId serves two purposes
- to control generalisation when we have -XMonoLocalBinds
- to check for floatability when dealing with (static e)
These are related, but not the same, and they'd becomme confused.
Trac Trac #13804 showed this up via an example like this:
f periph = let sr :: forall a. [a] -> [a] sr = if periph then reverse else id sr2 = sr -- The question: is sr2 generalised? -- It should be, because sr has a type sig -- even though it has periph free in (sr2 [True], sr2 "c")
Here sr2 should be generalised, despite the free var 'periph'
in 'sr' because 'sr' has a closed type signature.
I documented all this very carefully this time, in TcRnTypes:
Note [Meaning of IdBindingInfo] Note [Bindings with closed types: ClosedTypeId]