On Oct 6, 2010, at 9:36 AM, John McCall wrote:
>
> On Oct 6, 2010, at 7:55 AM, Douglas Gregor wrote:
>
>>
>> On Sep 27, 2010, at 5:56 AM, John McCall wrote:
>>
>>> On Sep 20, 2010, at 6:30 PM, Douglas Gregor wrote:
>>>> I'd really rather not add a new expression type just for the purpose of
>>>> type-checking __is_convertible_to. How about we synthesize appropriate
>>>> expressions instead, e.g., a DeclRefExpr to a synthesized variable, or a
>>>> CallExpr to a synthesized function with an appropriate return type?
>>>
>>> Actually, I think there are several places in the AST where "opaque
>>> expression of fixed type and l-valueness" would be very convenient — mostly
>>> with copies, e.g. with synthesized array copies, caught-exception copies,
>>> etc.
>>
>> Synthesized array copies always have a parameter to hang on to.
>
> Right, but we end up synthesizing array index variables. I'm suggesting this
> as a general alternative to making a reference to a synthesized variable.
>
>> I'm on the fence. I don't like having expression nodes that don't represent
>> ideas in the language, but there is some messiness in the expression
>> synthesis we do in Sema.
>
> I'm just saying that there are several places in the language where values
> come from no intrinsically-expressible place, and while it's obviously always
> possible to fake up expressions with the right properties in terms of
> spurious variables, I'm not sure I accept that that's somehow inherently
> cleaner than having a pair of expressions for "this opaque value comes from
> some place that the context knows about" and "the sub-expression needs these
> opaque values to work". I'm basically suggesting specialized lambdas. :)
> They'd definitely be constrained to a few places in the AST, though.
And.... John convinced me. We do want to add an OpaqueValueExpr (or whatever
name fits) that has type + value classification information. It can be used in
various places in Sema and the AST: for type checking when we need a
placeholder expression, for storing placeholder arguments for block-variable
copies and catch-variable copies, for the placeholder in an implicit
copy-construction of an array, and for the dreaded GNU x ? : y extension.
- Doug
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