Pattern synonym parse error should recommend enabling extension

Currently, if you try to use pattern synonyms in a module without -XPatternSynonyms, you can get a very uninformative parse error. For example:

module X where
import Data.Text (pattern Y)
x = 3

Yields, when compiled on GHC 7.10,

test.hs:3:27: parse error on input "Y"

It would be helpful if in addition to the error, the message suggested to the user to enable the PatternSynonyms extension, as many other error messages already do.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 3

added Tbug Trac import labels

That sounds very sensible and do-able. Would anyone like to offer a patch? We have precedents; e.g. a sensible error when people use forall without RankNTypes.

Simon

added newcomer label

assigned to @trac-ruperthorlick

Trac metadata

Trac field	Value
Version	7.10.3 → 8.1
Test case	- → parser/should_fail/T12429

Trac metadata

Trac field	Value
Differential revisions	- → D3261

mentioned in commit 4b1f0721

closed

changed milestone to %8.2.1

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added Plow label