Adding a -Wunused-value warning for expressions with side effects used in an unevaluated expression context, such as sizeof(), or decltype(). Also adds a similar warning when the expression passed to typeid() *is* evaluated, since it is equally likely that the user would expect the expression operand to be unevaluated in that case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224465 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index 22cc925c25..ef38fec2d9 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -1290,6 +1290,13 @@ Sema::CreateGenericSelectionExpr(SourceLocation KeyLoc,
     ControllingExpr = result.get();
   }
 
+  // The controlling expression is an unevaluated operand, so side effects are
+  // likely unintended.
+  if (ActiveTemplateInstantiations.empty() &&
+      ControllingExpr->HasSideEffects(Context, false))
+    Diag(ControllingExpr->getExprLoc(),
+         diag::warn_side_effects_unevaluated_context);
+
   bool TypeErrorFound = false,
        IsResultDependent = ControllingExpr->isTypeDependent(),
        ContainsUnexpandedParameterPack
@@ -3525,6 +3532,12 @@ bool Sema::CheckUnaryExprOrTypeTraitOperand(Expr *E,
     return true;
   }
 
+  // The operand for sizeof and alignof is in an unevaluated expression context,
+  // so side effects could result in unintended consequences.
+  if ((ExprKind == UETT_SizeOf || ExprKind == UETT_AlignOf) &&
+      ActiveTemplateInstantiations.empty() && E->HasSideEffects(Context, false))
+    Diag(E->getExprLoc(), diag::warn_side_effects_unevaluated_context);
+
   if (CheckObjCTraitOperandConstraints(*this, ExprTy, E->getExprLoc(),
                                        E->getSourceRange(), ExprKind))
     return true;