[commit: ghc] master: rts: Disable fallthrough attribute when compiling with Clang (6a71add)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/6a71add81d8f30b0caca0e869fb8e35787881c87/ghc

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

commit 6a71add81d8f30b0caca0e869fb8e35787881c87
Author: Ben Gamari <ben at smart-cactus.org>
Date:   Mon Dec 10 21:48:51 2018 -0500

    rts: Disable fallthrough attribute when compiling with Clang
    
    Apparently clang doesn't enable implicitly fallthrough warnings by default
    http://llvm.org/viewvc/llvm-project?revision=167655&view=revision when compiling
    C and the attribute cause warnings of their own (#16019).


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

6a71add81d8f30b0caca0e869fb8e35787881c87
 includes/Stg.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/includes/Stg.h b/includes/Stg.h
index 9b54526..57cb00d 100644
--- a/includes/Stg.h
+++ b/includes/Stg.h
@@ -197,7 +197,11 @@
 #endif
 
 /* Used to mark a switch case that falls-through */
-#if (defined(__GNUC__) && __GNUC__ >= 7) || defined(__clang__)
+#if (defined(__GNUC__) && __GNUC__ >= 7)
+// N.B. Don't enable fallthrough annotations when compiling with Clang.
+// Apparently clang doesn't enable implicitly fallthrough warnings by default
+// http://llvm.org/viewvc/llvm-project?revision=167655&view=revision
+// when compiling C and the attribute cause warnings of their own (#16019).
 #define FALLTHROUGH GNU_ATTRIBUTE(fallthrough)
 #else
 #define FALLTHROUGH ((void)0)