Clearify status of ImpredicativeTypes Extension

There seem to be many statements about this extension and its problems floating around in the internet. But a user of GHC will first fall into a possibly large number of traps and problems before he finds them.

I think it would help, if the GHC manual https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/other-type-extensions.html#impredicative-polymorphism and the compiler warned about this extension (instead of suggesting it!).

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
Test case
Differential revisions
BlockedBy
Related
Blocking
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Operating system
Architecture

mentioned in issue #9618 (closed)

changed weight to 5

added Tbug Trac import documentation labels

I agree with j80JjBjVNRMajmA, and see also #9618 (closed). Meanwhile #10281 (closed) is in part about expanding the documentation of ImpredicativeTypes slightly.

Trac metadata

Trac field	Value
Related	→ #9618 (closed), #10281 (closed)

mentioned in commit c715166f

closed

Done! Thanks.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

changed milestone to %8.0.1

added Pnormal label