Improve exhaustive checking for guards in pattern bindings and MultiIf.

Previously we didn't do exhaustive checking on MultiIf expressions
and guards in pattern bindings.

We can construct the `LMatch` directly from GRHSs or [LHsExpr]
(MultiIf's alts) then feed it to checkMatches, without construct the
MatchGroup and using function `matchWrapper`.

Signed-off-by: HE, Tao <sighingnow@gmail.com>

Test Plan: make test TEST="T14773a T14773b"

Reviewers: bgamari, RyanGlScott, simonpj

Reviewed By: bgamari, simonpj

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14773

Differential Revision: https://phabricator.haskell.org/D4400

1 files changed, 16 insertions, 0 deletions

diff --git a/docs/users_guide/8.6.1-notes.rst b/docs/users_guide/8.6.1-notes.rst
index 70c3b0c10d..f5f2a5a341 100644
--- a/docs/users_guide/8.6.1-notes.rst
+++ b/docs/users_guide/8.6.1-notes.rst
@@ -31,6 +31,22 @@ Language
   The grammar is invalid in Haskell2010. Previously it could be compiled successfully
   without ``GADTs``. As of GHC 8.6.1, this is a parse error.
 
+- Incomplete patterns warning :ghc-flag:`-Wincomplete-patterns` is extended to
+  guards in pattern bindings and ``if`` alternatives of :extension:`MultiWayIf`.
+  For instance, consider the following, ::
+
+      foo :: Bool -> Int
+      foo b = if | b -> 1
+
+  In GHC 8.6.1, it will raise the warning: ::
+
+      <interactive>:2:12: warning: [-Wincomplete-patterns]
+          Pattern match(es) are non-exhaustive
+          In a multi-way if alternative:
+              Guards do not cover entire pattern space
+
+  See :ghc-ticket:`14773`.
+
 Compiler
 ~~~~~~~~