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

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/hsSyn/HsPat.hs b/compiler/hsSyn/HsPat.hs
index faefb84203..6f65487411 100644
--- a/compiler/hsSyn/HsPat.hs
+++ b/compiler/hsSyn/HsPat.hs
@@ -34,7 +34,7 @@ module HsPat (
         patNeedsParens, parenthesizePat,
         isIrrefutableHsPat,
 
-        collectEvVarsPats,
+        collectEvVarsPat, collectEvVarsPats,
 
         pprParendLPat, pprConArgs
     ) where