Fix PR2279 part C: shifts don't perform the UACs, thanks to Neil

for pointing this out.

llvm-svn: 50624

clang/lib/Lex/PPExpressions.cpp

@@ -409,9 +409,11 @@ static bool EvaluateDirectiveSubExpr(llvm::APSInt &LHS, unsigned MinPrec,
     assert(PeekPrec <= ThisPrec && "Recursion didn't work!");
     
     // Usual arithmetic conversions (C99 6.3.1.8p1): result is unsigned if
-    // either operand is unsigned.  Don't do this for x and y in "x ? y : z".
+    // either operand is unsigned.  Don't do this for x and y in "x ? y : z" or
+    // for shifts.
     llvm::APSInt Res(LHS.getBitWidth());
-    if (Operator != tok::question) {
+    if (Operator != tok::question && Operator != tok::lessless &&
+        Operator != tok::greatergreater) {
       Res.setIsUnsigned(LHS.isUnsigned()|RHS.isUnsigned());
       // If this just promoted something from signed to unsigned, and if the
       // value was negative, warn about it.

clang/test/Preprocessor/expr_usual_conversions.c

@@ -6,3 +6,8 @@
 foo
 #endif
 
+// Shifts don't want the usual conversions: PR2279
+#if (2 << 1U) - 30 >= 0
+#error
+#endif
+