[cfe-dev] Function Instantiation: Where? Or is it?

[Larisse Voufo]

Ow. Right. Thanks. I'll give that a show and let you know...


On Tue, Feb 1, 2011 at 4:37 PM, Douglas Gregor <dgregor at apple.com> wrote:

>
> On Feb 1, 2011, at 12:42 PM, Larisse Voufo wrote:
>
>
>
> On Tue, Feb 1, 2011 at 3:01 PM, Douglas Gregor <dgregor at apple.com> wrote:
>
>>
>> On Feb 1, 2011, at 11:45 AM, Larisse Voufo wrote:
>>
>> Let me see if I can rephrase this problem...
>> At this point, instantiation is no longer an issue. The issue is managing
>> new DeclContext's.
>>
>> Take the following definition of a concept:
>>
>> concept A<typename T> {
>>     void f(int) { }
>> }
>>
>> This definition is parsed into a ConceptDecl  object *ContextA* where
>> ConceptDecl is a DeclContext as well as a TemplateDecl.
>> When the concept is used on a generic function such as:
>>
>> template<typename T>
>> requires A<T>
>> void func(T a) {
>>     f(a);
>> }
>>
>> at the call f(a),  the lookup process finds the definition of f() in the
>> DeclContext *ContextA*.
>> Hence, the definition of func() is accepted.
>> Let's refer to the found definition as *Fn*.
>>
>> Now, when func() is used in the main, the lookup and instantiation-related
>> processes are still finding and using that same definition of f(): Fn.
>>
>>
>> That's wrong. You need to map from the concept's function f() to the
>> corresponding function f() in the concept map.
>>
>> - Doug
>>
>
> Perhaps I should've phrased that this is the sample case I'm examining at
> the moment, not the defacto way that concepts should work. The idea above
> should generalize to any DeclContext, whether they are  ConceptDecls or not.
>
> For instance, forget concepts, call  "concept A<typename T>" My_DeclContext
> and replace everything as follows:
>
> My_DeclContext A {
>     void f(int) { }
> }
> //This definition is parsed into a SomeDecl  object *ContextA* where
> SomeDecl is a DeclContext.
>
> template<...>
> *lookup_in (A*)
> void func(T a) {
>     f(a);
> }
>
> Again, the observations from the previous example should naturally
> translate to this case, and they are the result of an intensive debugging
> session (overnight)...
>
> *If Fn is found and used throughout the compilation, how come it gets lost
> at link time? *
> My interest is that: if we can get this to work, then I'll know for sure
> that I understand the way that lookup and instantiation work in clang, with
> respect to added declaration contexts.
>
>
> Fn is in a concept, so it can't be instantiated without instantiating the
> concept itself first to produce an instantiated Fn declaration. Then you can
> go ahead and instantiate the body of the instantiated Fn declaration. Step
> through how a member function of a class template gets instantiated:
>
> template<typename T>
> struct A {
> void f(int);
> };
>
> It doesn't make any sense to instantiate A<T>::f(int) without knowing what
> T is, but that's what you're trying to do with concepts.
>
> I'm concerned that we are perhaps getting confused from thinking about this
> problem from completely different perspective. Marcin was also quite ahead
> of me on this. :)
> Please, just note that I am not particularly thinking of concepts at the
> moment... I want to very basically understand why the case above would cause
> the error I am getting, when the only major change I made was in deviating
> the lookup process for f() so that it is looked up in *ContextA *-- using
> LookupQualifiedName(...), rather than in the global scope*...
> *
>
>
> The change you made means that the function declaration "f" that's being
> referenced from the instantiation of "func" is still dependent on a
> template, even though this is non-template code (after instantiation).
>
> - Doug
>
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