[cfe-dev] Working with ConceptClang: Binding input arguments to functions.

[Larisse Voufo]

On Fri, Apr 22, 2011 at 11:51 AM, Douglas Gregor <dgregor at apple.com> wrote:

>
> On Apr 22, 2011, at 2:05 AM, Larisse Voufo wrote:
>
> Here's a basic example I've been trying to get to work:
>
> explicit concept A<typename T> {
>     typename AType;
>
>     AType h(AType a) {
>         return a;
>     }
>
>     AType f(AType a) {
>         AType b;
>         return b;
>     }
> }
>
> concept_map A<int> {
>     typedef char AType;
> }
>
> template<typename T>
> requires (A<T>)
> void func(T a, AType b) {
>     AType x = h(b);         // <-- Instantiation DOES NOT WORK
>     AType y = f(b);          // <-- Instantiation WORKS
> }
>
> int main(int argc, char **argv)
> {
>     func<int>(10, 'a');      // Fails with declaration for 'x', for passes
> with declaration of 'y'.
> }
>
>
> It appears that during the instantiation of *func<int>()*, the value for
> input parameter '*b*' for '*func()*'
> can be looked up no problem. However, that of '*a*' for associated
> function '*h()*' cannot.
> Instead, a call to *clang::LocalInstantiationScope::getInstantiationOf()*fails at runtime on the assertion:
>
> *(D->isInvalidDecl() && "declaration was not instantiated in this scope!")
> *.
>
> I have been tracing through the procedures for building, transforming, and
> rebuilding Call Expressions -- as well as instantiating functions, noting
> each
> execution of *clang::LocalInstantiationScope::getInstantiationOf()* and *
> clang::LocalInstantiationScope::InstantiatedLocal()*...
>
> Still, I can't see what needs to be done differently for calls to *h()*from calls to
> *func<int>()*. Hence...
>
> Question:
> When/how exactly are input arguments bound to functions?
>
>
> That's a runtime issue. All the front-end does is enforce the calling
> convention on both sides of the call.
>

And how does it do that? Exactly, when is *func<int>*'s '*b*' supposed to
meet *h*'s '*a*'?
Just as... When do the *main*'s *10* and '*a*' meet *func<int>*'s '*a*' and
'*b*' ?

>
> Why would the current procedure fail to bind the value of '*b*' from the
> context of *func<int>* to the
> parameter '*a*' of *h* ?
>
>
> They are in completely different contexts. func<int>'s b is completely
> disjoint from h's a. And, in fact, they might not be the same thing, since
> passing 'b' as an argument to h() implies the creation of a temporary.
>
> The only way I could see this issue coming up is if you're doing some kind
> of inlining of A<AType>::h into func<int> as part of instantiation. If so, I
> recommend against doing that: inlining will be handled by later optimization
> passes, and tangling the func<int> instantiation with the A<AType>::h
> instantiation is going to cause more problems in the front end than it
> solves.
>

Hmm... How would the call be handled if *h* was a global function -- a
specialization of some template, not associated with any concept?
I am not trying to achieve any inlining here, I just want to pass in the
arguments as I would any inner function call (though thanks for the tip)...

Thanks,
-- Larisse.




>
> - Doug
>
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