[commit: ghc] master: exprIsBottom: Make use of isEmptyTy (#10186) (7062ebe)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/7062ebe0ce92c191d87e993bd2497275976b9452/ghc

>---------------------------------------------------------------

commit 7062ebe0ce92c191d87e993bd2497275976b9452
Author: Joachim Breitner <mail at joachim-breitner.de>
Date:   Mon Mar 23 22:25:24 2015 +0100

    exprIsBottom: Make use of isEmptyTy (#10186)
    
    Any expression with of empty type is necessary bottom, so we can use
    that here. No effects known, but it is the right thing to do and
    validate, so lets do it.
    
    Differential Revision: https://phabricator.haskell.org/D754


>---------------------------------------------------------------

7062ebe0ce92c191d87e993bd2497275976b9452
 compiler/coreSyn/CoreLint.hs  | 2 +-
 compiler/coreSyn/CoreUtils.hs | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/compiler/coreSyn/CoreLint.hs b/compiler/coreSyn/CoreLint.hs
index d5b031a..81e5618 100644
--- a/compiler/coreSyn/CoreLint.hs
+++ b/compiler/coreSyn/CoreLint.hs
@@ -661,7 +661,7 @@ lintCoreExpr e@(Case scrut var alt_ty alts) =
      ; when (null alts) $
      do { checkL (not (exprIsHNF scrut))
           (ptext (sLit "No alternatives for a case scrutinee in head-normal form:") <+> ppr scrut)
-        ; checkL (exprIsBottom scrut || isEmptyTy (exprType scrut))
+        ; checkL (exprIsBottom scrut)
           (ptext (sLit "No alternatives for a case scrutinee not known to diverge for sure:") <+> ppr scrut)
         }
 
diff --git a/compiler/coreSyn/CoreUtils.hs b/compiler/coreSyn/CoreUtils.hs
index ba40f25..f400ebc 100644
--- a/compiler/coreSyn/CoreUtils.hs
+++ b/compiler/coreSyn/CoreUtils.hs
@@ -694,6 +694,10 @@ expensive.
 -}
 
 exprIsBottom :: CoreExpr -> Bool
+-- If the type only contains no elements besides bottom, then this expressions,
+-- well, bottom.
+exprIsBottom e | isEmptyTy (exprType e) = True
+-- Otherwise see if this is a bottoming id applied to enough arguments
 exprIsBottom e
   = go 0 e
   where