[cfe-dev] Who owns declarations?

[Douglas Gregor]

On Dec 22, 2008, at 11:11 AM, Douglas Gregor wrote:
> On Dec 22, 2008, at 8:05 PM, Sebastian Redl wrote:
>
>> Douglas Gregor wrote:
>>>
>>> On Dec 22, 2008, at 7:38 PM, Sebastian Redl wrote:
>>>
>>>>
>>>> By the way, why does CXXConditionalDeclExpr derive from  
>>>> DeclRefExpr?
>>>
>>> I believe it's because IfStmt's condition is an Expr*, so
>>> CXXConditionalDeclExpr is an Expr that also declares a variable.
>>> The derivation from DeclRefExpr allows the rest of semantic
>>> analysis (that knows how to treat a DeclRefExpr as a boolean
>>> expression) to deal with the declaration.
>>>
>>>> That's counter-intuitive from several viewpoints.
>>>
>>>
>>> How so?
>>>
>> A CXXConditionalDeclExpr declares a variable, it doesn't reference  
>> it.
>
> It does both. We're talking about something like
>
> 	if (int x = foo()) {
> 	}
>
> where everything in the parentheses is the CXXConditionalDeclExpr. It
> declares the variable x, and then acts as a reference to the  
> variable x.
>
>> That's the most important difference between the two, and it doesn't
>> lend itself to using derivation - note that CXXConditionalDeclExpr
>> doesn't have serialization implemented, precisely because it owns
>> the Decl and DeclRefExpr doesn't, which causes confusion.
>
>
> Perhaps what we really need is not the CXXConditionalDeclExpr class,
> but the ability for DeclRefExpr to own its declaration (based on some
> flag, which could just be the expression class itself). That would
> also be useful, e.g., when we have to build an OverloadedFunctionDecl
> on-the-fly that will be owned by a DeclRefExpr, say, to retain the
> results of name lookup in a template.


So, in talking about this over lunch, we decided that  
CXXConditionalDeclExpr isn't the best way to model this (which you  
already knew <g>). The convincing argument is that the  
CXXConditionalDeclExpr might not even be the "If" expression itself,  
if it ends up being a class type with a conversion operator, e.g.,

	struct X {
		operator int();
	};

	if (X x = foo()) {
		// The expression for the If will be some kind of implicit call to  
X::operator int(), whose argument is the CXXConditionalDeclExpr
	}

It would be better for the IfStmt to have an optional VarDecl in it,  
and remove CXXConditionalDeclExpr.

It'd also be nice if we could handle the code above :)

	- Doug