Don't warn about unused values in ternary ?: expressions unless both the LHS and RHS are "unused" (side-effect free).

Patch by Justin Bogner!  Fixes PR 8282.

llvm-svn: 126779

2 files changed, 16 insertions, 5 deletions

diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 8e44c073682..930457c249c 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -1404,13 +1404,15 @@ bool Expr::isUnusedResultAWarning(SourceLocation &Loc, SourceRange &R1,
     return false;
 
   case ConditionalOperatorClass: {
-    // The condition must be evaluated, but if either the LHS or RHS is a
-    // warning, warn about them.
+    // If only one of the LHS or RHS is a warning, the operator might
+    // be being used for control flow. Only warn if both the LHS and
+    // RHS are warnings.
     const ConditionalOperator *Exp = cast<ConditionalOperator>(this);
-    if (Exp->getLHS() &&
-        Exp->getLHS()->isUnusedResultAWarning(Loc, R1, R2, Ctx))
+    if (!Exp->getRHS()->isUnusedResultAWarning(Loc, R1, R2, Ctx))
+      return false;
+    if (!Exp->getLHS())
       return true;
-    return Exp->getRHS()->isUnusedResultAWarning(Loc, R1, R2, Ctx);
+    return Exp->getLHS()->isUnusedResultAWarning(Loc, R1, R2, Ctx);
   }
 
   case MemberExprClass:
diff --git a/clang/test/Sema/warn-unused-value.c b/clang/test/Sema/warn-unused-value.c
index 876eb9e4823..95cd8fb7040 100644
--- a/clang/test/Sema/warn-unused-value.c
+++ b/clang/test/Sema/warn-unused-value.c
@@ -72,6 +72,15 @@ int test_logical_bar() {
   return x;
 }
 
+// PR8282
+void conditional_for_control_flow(int cond, int x, int y)
+{
+    cond? y++ : x; // no-warning
+    cond? y : ++x; // no-warning
+    cond? (x |= y) : ++x; // no-warning
+    cond? y : x; // expected-warning {{expression result unused}}
+}
+
 struct s0 { int f0; };
 
 void f0(int a);