Implement Or Patterns (Proposal 0522)wip/or-pats

This commit introduces a language extension, `OrPatterns`, as described in proposal 0522.

It extends the syntax by the production
`pat -> (one of pat1, ..., patk)`.
The or-pattern `pat` succeeds iff one of the patterns `pat1`, ..., `patk` succeed, in this order.

Currently, or-patterns cannot bind variables. They are still of great use as they discourage the use of wildcard patterns in favour of writing out all "default" cases explicitly:

```
isIrrefutableHsPat pat = case pat of
  ...
  (one of WildPat{}, VarPat{}, LazyPat{})
      = True
  (one of PArrPat{}, ConPatIn{}, LitPat{}, NPat{}, NPlusKPat{}, ListPat{})
      = False
```

This makes code safer where data types are extended now and then - just like GHC's `Pat` in the example when adding the new `OrPat` constructor. This would be catched by `-fwarn-incomplete-patterns`, but not when a wildcard pattern was used.

- Update submodule haddock.

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 2ad09bc7c6..c544b398a7 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -3703,6 +3703,7 @@ xFlagsDeps = [
   depFlagSpec' "NullaryTypeClasses"           LangExt.NullaryTypeClasses
     (deprecatedForExtension "MultiParamTypeClasses"),
   flagSpec "NumDecimals"                      LangExt.NumDecimals,
+  flagSpec "OrPatterns"                       LangExt.OrPatterns,
   depFlagSpecOp "OverlappingInstances"        LangExt.OverlappingInstances
     setOverlappingInsts
     "instead use per-instance pragmas OVERLAPPING/OVERLAPPABLE/OVERLAPS",