Turn off implicit truncation warning for compound assignment to bitfields; it might be reasonable in some cases, but it clearly doesn't make sense in some cases, like the included testcase.

<rdar://problem/10238797>, part 2.

llvm-svn: 149095

clang/lib/Sema/SemaChecking.cpp

@@ -4053,8 +4053,8 @@ void AnalyzeImplicitConversions(Sema &S, Expr *OrigE, SourceLocation CC) {
     if (BO->isComparisonOp())
       return AnalyzeComparison(S, BO);
 
-    // And with assignments and compound assignments.
-    if (BO->isAssignmentOp())
+    // And with simple assignments.
+    if (BO->getOpcode() == BO_Assign)
       return AnalyzeAssignment(S, BO);
   }
 

clang/test/Sema/constant-conversion.c

@@ -71,4 +71,6 @@ void test7() {
 
 	f.twoBits1 = ~1; // expected-warning {{implicit truncation from 'int' to bitfield changes value from -2 to 2}}
 	f.twoBits2 = ~2; // expected-warning {{implicit truncation from 'int' to bitfield changes value from -3 to 1}}
+	f.twoBits1 &= ~1; // no-warning
+	f.twoBits2 &= ~2; // no-warning
 }