Re: r224465 - 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.

[Stephan Bergmann]

On 12/19/2014 02:20 PM, Timur Iskhodzhanov wrote:
I believe this is a false positive:
-----------------------------------------
   #include <typeinfo>
   #include <vector>
   struct A {
     virtual ~A();
   };

   const char *foo(const std::vector<A*> &v) {
     const std::type_info &t = typeid(*v[0]);
     return t.name <http://t.name>();
   }
-----------------------------------------

This warning seems to be pretty annoying as it's on by default.

Is it possible to revert/disable and reland it gradually, making sure we
don't get false positives along the way?
or at least put it under a separate flag for the time being?

Yes, this annoyed me too when building LibreOffice, quite a number of 
places there that use typeid(*...) dereferencing complex expressions 
yielding polymorphic pointers, so I hacked my local Clang for now with

Index: lib/Sema/SemaExprCXX.cpp
===================================================================
--- lib/Sema/SemaExprCXX.cpp    (revision 224504)
+++ lib/Sema/SemaExprCXX.cpp    (working copy)
@@ -449,10 +449,18 @@
   else if (ActiveTemplateInstantiations.empty() &&
            E->HasSideEffects(Context, WasEvaluated)) {
     // The expression operand for typeid is in an unevaluated expression
-    // context, so side effects could result in unintended consequences.
-    Diag(E->getExprLoc(), WasEvaluated
-                              ? diag::warn_side_effects_typeid
-                              : diag::warn_side_effects_unevaluated_context);
+    // context, so side effects could result in unintended consequences (but
+    // suppress warnings when dereferencing a pointer [TODO: to non-final class
+    // type], so evaluation of the expression, including side effects and all,
+    // is likely expected after all).
+    UnaryOperator *Op;
+    if (!WasEvaluated ||
+        ((Op = dyn_cast<UnaryOperator>(E->IgnoreParenImpCasts())) &&
+         Op->getOpcode() == UO_AddrOf)) {
+      Diag(E->getExprLoc(), WasEvaluated
+                                ? diag::warn_side_effects_typeid
+                                : diag::warn_side_effects_unevaluated_context);
+    }
   }

   return new (Context) CXXTypeidExpr(TypeInfoType.withConst(), E,

Stephan
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