[cfe-commits] r138715 - in /cfe/trunk: lib/Sema/SemaExpr.cpp test/CXX/special/class.temporary/p1.cpp

[Johannes Schaub]

John McCall wrote:

> On Aug 27, 2011, at 4:15 PM, Johannes Schaub wrote:
>> John McCall wrote:
>>> Modified: cfe/trunk/lib/Sema/SemaExpr.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-
>> 
project/cfe/trunk/lib/Sema/SemaExpr.cpp?rev=138715&r1=138714&r2=138715&view=diff
>>> 
>> 
==============================================================================
>>> --- cfe/trunk/lib/Sema/SemaExpr.cpp (original) +++
>>> cfe/trunk/lib/Sema/SemaExpr.cpp Sat Aug 27 17:06:17 2011 @@ -443,9
>>> +443,15 @@
>>>   if (Ty->isSpecificBuiltinType(BuiltinType::Float))
>>>     E = ImpCastExprToType(E, Context.DoubleTy, CK_FloatingCast).take();
>>> 
>>> -  // C++ includes lvalue-to-rvalue conversion as a default argument
>>> -  // promotion.  If we have a gl-value, initialize a temporary.
>>> -  if (getLangOptions().CPlusPlus && E->isGLValue()) {
>>> +  // C++ performs lvalue-to-rvalue conversion as a default argument
>>> +  // promotion.  If we still have a gl-value after usual unary
>>> +  // conversion, we must have an l-value of class type, so we need to
>>> +  // initialize a temporary.  For compatibility reasons, however, we
>>> +  // don't want to do this in unevaluated contexts; otherwise we
>>> +  // reject metaprograms which work by passing uncopyable l-values to
>>> +  // variadic functions.
>> 
>> 4.1p2 of the C++11 FDIS says:
>> 
>> "When an lvalue-to-rvalue conversion occurs in an unevaluated operand or
>> a subexpression thereof (Clause 5) the value contained in the referenced
>> object is not accessed. Otherwise ...".
>> 
>> C++03 is much less precise on the entire conversion wrt class types, but
>> said if the expression occurs within the operand of a sizeof "the value
>> contained in the referenced object is not accessed". I think the FDIS
>> text will be a good reference to put on that comment.
> 
> I'm not sure how this applies.  I agree that the lvalue-to-rvalue
> conversion, if well-formed, does not access the object if it appears in an
> unevaluated
> context.  However, the conversion is not necessarily well-formed, and
> normally that's checked even in unevaluated contexts.
> 

Not only is the value not accessed at runtime, but there is also no copy 
initialization done at all. That's what is explicit in the C++0x FDIS. 
That's the purpose of "Otherwise [if we are in an evaluated context], ...".

There is no such rule that says similar things for access checking when 
calling a member function. So you of course still have to check access when 
calling a member function. 

That's much the same treatment as what has been done to creation of a 
temporary as the result of analyzing the toplevel expression of a decltype:

    struct A { private: ~A(); };
    A f();
    int main() { 
      // well-formed
      decltype(f()); 
      // ill-formed
      f();
    }

Just like for the lvalue to rvalue conversion, in that case we too have an 
explicit rule that in that particular case, no temporary is created and to 
be destroyed.