[commit: ghc] ghc-8.2: compiler: Do not look up fail in RnExpr if bind pattern is irrefutible. (56a4863)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-8.2
Link       : http://ghc.haskell.org/trac/ghc/changeset/56a4863b25687319a07db596bd47d724456317a5/ghc

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commit 56a4863b25687319a07db596bd47d724456317a5
Author: Aaron Friel <mayreply at aaronfriel.com>
Date:   Thu May 11 15:41:22 2017 -0400

    compiler: Do not look up fail in RnExpr if bind pattern is irrefutible.
    
    Adds a check in `rnStmt`, in sub-expr `getFailFunction`, to determine if
    the pattern of a bind statement is irrefutible. If so, skip looking up
    the `fail` name.
    
    Reviewers: austin, bgamari
    
    Reviewed By: bgamari
    
    Subscribers: rwbarton, thomie
    
    GHC Trac Issues: #13649
    
    Differential Revision: https://phabricator.haskell.org/D3553
    
    (cherry picked from commit 2fcb5c5c3f6c5a5936eeb5dc07b476e5737f12ad)


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56a4863b25687319a07db596bd47d724456317a5
 compiler/rename/RnExpr.hs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/compiler/rename/RnExpr.hs b/compiler/rename/RnExpr.hs
index 4e9192c..fe3d308 100644
--- a/compiler/rename/RnExpr.hs
+++ b/compiler/rename/RnExpr.hs
@@ -824,6 +824,10 @@ rnStmt ctxt rnBody (L loc (BindStmt pat body _ _ _)) thing_inside
 
         ; xMonadFailEnabled <- fmap (xopt LangExt.MonadFailDesugaring) getDynFlags
         ; let getFailFunction
+                -- If the pattern is irrefutible (e.g.: wildcard, tuple,
+                -- ~pat, etc.) we should not need to fail.
+                | isIrrefutableHsPat pat
+                                    = return (noSyntaxExpr, emptyFVs)
                 -- For non-monadic contexts (e.g. guard patterns, list
                 -- comprehensions, etc.) we should not need to fail.
                 -- See Note [Failing pattern matches in Stmts]