[Sema] Warn when shifting a negative value.

Example:
 % ./clang -Wshift-negative-value emit.c
emit.c:3:14: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
  int a = -1 << 3;
          ~~ ^
1 warning generated.

PR:		24026
Differential Revision:	 http://reviews.llvm.org/D10938
Reviewed by:	rsmith

llvm-svn: 241478

1 files changed, 3 insertions, 2 deletions

diff --git a/clang/test/Sema/shift.c b/clang/test/Sema/shift.c
index d3555448851..07c5fe5280e 100644
--- a/clang/test/Sema/shift.c
+++ b/clang/test/Sema/shift.c
@@ -39,7 +39,8 @@ void test() {
   i = 1 << (WORD_BIT - 2);
   i = 2 << (WORD_BIT - 1); // expected-warning {{bits to represent, but 'int' only has}}
   i = 1 << (WORD_BIT - 1); // expected-warning {{sets the sign bit of the shift expression}}
-  i = -1 << (WORD_BIT - 1);
+  i = -1 << (WORD_BIT - 1); // expected-warning {{shifting a negative signed value is undefined}}
+  i = -1 << 0; // expected-warning {{shifting a negative signed value is undefined}}
   i = 0 << (WORD_BIT - 1);
   i = (char)1 << (WORD_BIT - 2);
 
@@ -48,7 +49,7 @@ void test() {
   u = 5U << (WORD_BIT - 1);
 
   long long int lli;
-  lli = INT_MIN << 2; // expected-warning {{bits to represent, but 'int' only has}}
+  lli = INT_MIN << 2; // expected-warning {{shifting a negative signed value is undefined}}
   lli = 1LL << (sizeof(long long) * CHAR_BIT - 2);
 }