Fix #15385 by using addDictsDs in matchGuards

Summary:
When coverage checking pattern-matches, we rely on the call
sites in the desugarer to populate the local dictionaries and term
evidence in scope using `addDictsDs` and `addTmCsDs`. But it turns
out that only the call site for desugaring `case` expressions was
actually doing this properly. In another part of the desugarer,
`matchGuards` (which handles pattern guards), it did not update the
local dictionaries in scope at all, leading to #15385.

Fixing this is relatively straightforward: just augment the
`BindStmt` case of `matchGuards` to use `addDictsDs` and `addTmCsDs`.
Accomplishing this took a little bit of import/export tweaking:

* We now need to export `collectEvVarsPat` from `HsPat.hs`.
* To avoid an import cycle with `Check.hs`, I moved `isTrueLHsExpr`
  from `DsGRHSs.hs` to `DsUtils.hs`, which resides lower on the
  import chain.

Test Plan: make test TEST=T15385

Reviewers: simonpj, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15385

Differential Revision: https://phabricator.haskell.org/D4968

2 files changed, 27 insertions, 4 deletions

diff --git a/testsuite/tests/pmcheck/should_compile/T15385.hs b/testsuite/tests/pmcheck/should_compile/T15385.hs
new file mode 100644
index 0000000000..dedf6c1553
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T15385.hs
@@ -0,0 +1,21 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE MultiWayIf #-}
+{-# LANGUAGE TypeOperators #-}
+{-# OPTIONS_GHC -Wincomplete-patterns #-}
+module T15385 where
+
+import Data.Type.Equality
+
+data T a where
+  TInt  :: T Int
+  TBool :: T Bool
+
+f1, f2 :: a :~: Int -> T a -> ()
+f1 eq t
+  | Refl <- eq
+  = case t of
+      TInt -> ()
+f2 eq t
+  = if |  Refl <- eq
+       -> case t of
+            TInt -> ()
diff --git a/testsuite/tests/pmcheck/should_compile/all.T b/testsuite/tests/pmcheck/should_compile/all.T
index e382e3fda4..4030b0609a 100644
--- a/testsuite/tests/pmcheck/should_compile/all.T
+++ b/testsuite/tests/pmcheck/should_compile/all.T
@@ -44,25 +44,27 @@ test('T11276', compiler_stats_num_field('bytes allocated',
 
 test('T11303b', compiler_stats_num_field('bytes allocated',
   [(wordsize(64), 54373936, 10)]
-    # 2018-07-14: 54373936    INITIAL    
+    # 2018-07-14: 54373936    INITIAL
   ), compile, ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns +RTS -M1G -RTS'])
-  
+
 test('T11374', compiler_stats_num_field('bytes allocated',
   [(wordsize(64), 280144864, 10)]
     # 2018-07-14: 280144864   INITIAL
   ), compile, ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns +RTS -M1G -RTS'])
-  
+
 test('T11195', compiler_stats_num_field('bytes allocated',
   [(wordsize(64), 7852567480, 10)]
     # 2018-07-14: 7852567480   INITIAL
   ), compile, ['-package ghc -fwarn-incomplete-patterns -fwarn-overlapping-patterns +RTS -M2G -RTS'])
-  
+
 test('T11984', normal, compile,
     ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T14086', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T14098', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
+test('T15385', normal, compile,
+     ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 
 # Other tests
 test('pmc001', [], compile,