[cfe-dev] static constexpr member function of a local struct in a function template is instantiated too late

[Michael Park]

I've followed http://llvm.org/docs/DeveloperPolicy.html#one-off-patches and
http://llvm.org/docs/GettingStarted.html#sending-patches-with-git to send
an email to *cfe-commits* mailing list with a *git diff* as an attachment.
Compared to other emails though my seem to look different so I think I may
have done it wrong?


On 21 August 2014 21:40, Richard Smith <richard at metafoo.co.uk> wrote:

> On Thu, Aug 21, 2014 at 6:22 PM, Michael Park <mcypark at gmail.com> wrote:
>
>> Hi Richard, thanks for the reply!
>>
>> Funnily enough, the member functions of local classes that have deduced
>> return types actually don't run into this problem because the definition
>> gets instantiated eagerly in order to deduce the return type.
>> So the following actually works in clang++-3.4.2.
>>
>>  template <typename T>
>>> void F() {
>>>   struct Num {
>>>     static constexpr auto Get() { return 42; }
>>>   };
>>>   static_assert(Num::Get() == 42, "");
>>>
>>> }
>>> int main() {
>>>   F<int>();
>>> }
>>>
>>
>>  I tried the recommended approach of instantiating at the end of the
>> local class instead, like so:
>>
>> // DR1484 clarifies that the members of a local class are instantiated as
>>> part
>>> // of the instantiation of their enclosing entity.
>>> if (D->isCompleteDefinition() && D->isLocalClass()) {
>>>   SemaRef.InstantiateClass(D->getLocation(), Record, D, TemplateArgs,
>>>                            TSK_ImplicitInstantiation,
>>>                            /*Complain=*/true);
>>>   SemaRef.InstantiateClassMembers(D->getLocation(), Record, TemplateArgs,
>>>                                   TSK_ImplicitInstantiation);
>>>
>> *  SemaRef.PerformPendingInstantiations(/*LocalOnly=*/true);*
>>
>> }
>>
>>
>> Although there are *PeformPendingInstantiations *calls that should be
>> removed, it effectively will be a no-op so I just tried this out for now.
>> It actually passes *make test* and also passes the example above!
>>
>> But I worry that instantiating at the end of the local class is still too
>> late, because I run into the same issue if I bring the *static_assert* into
>> the body of the local struct.
>>
>> template <typename T>
>>
>> void F() {
>>
>>    struct Num {
>>>     static constexpr int Get() { return 42; }
>>>     static_assert(range == 42, "");
>>>   };
>>
>> }
>>
>>
> The core language is somewhat unclear on whether that's supposed to work.
> Note that it doesn't work if F is not a template. I think the standard
> committee will end up deciding that this case is ill-formed, so I'm happy
> for us to reject it for now.
>
> This case also works if I change the return type of *Get* to *auto*.
>>
>> Even I bring out the local struct to namespace scope, it still breaks.
>> But I think this might be a different bug. (In this case changing *Get*'s
>> return type to *auto* doesn't resolve the problem.)
>>
>
> =) Yep. See
> http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1626
>
>
>> struct Num {
>>>   static constexpr int Get() { return 42; }
>>>    static_assert(Get() == 42, "");
>>> };
>>
>>
>> P.S. Thanks so much for helping out.
>>
>>
>> On 20 August 2014 22:31, Richard Smith <richard at metafoo.co.uk> wrote:
>>
>>> On Tue, Aug 12, 2014 at 8:09 PM, Michael Park <mcypark at gmail.com> wrote:
>>>
>>>> Hello,
>>>>
>>>
>>> Hi and welcome! (And sorry for the delay.)
>>>
>>>
>>>> I'm new to clang development and was hoping to get some help in
>>>> tackling this bug that I found/filed:
>>>> http://llvm.org/bugs/show_bug.cgi?id=20625
>>>>
>>>> Example:
>>>>
>>>> template <typename T>
>>>>> void F() {
>>>>>   struct Num {
>>>>>     static constexpr int Get() { return 42; }
>>>>>   };
>>>>>   constexpr int n = Num::Get();
>>>>> }
>>>>> int main() {
>>>>>   F<int>();
>>>>> }
>>>>
>>>>
>>>> The summary of the issue is that the local struct *Num* and its static
>>>> constexpr function *Get* is not yet instantiated when we validate the
>>>> constexpr variable initialization of *n* during *F*'s instantiation.
>>>>
>>>> More details:
>>>>
>>>> *Num* and *Get* gets added to *PendingLocalImplicitInstantiations* in
>>>> *SubstStmt* at *SemaTemplateInstantiationDecl.cpp:3437*, and is
>>>> instantiated later by *PerformPendingInstantiations* at
>>>> *SemaTemplateInstantiationDecl.cpp:3458*. However, the validation of
>>>> the constexpr variable initialization of *n* happens in *SubstStmt* at
>>>> which point we don't yet have the definition of *Get* instantiated.
>>>>
>>>> I'm not exactly sure what the correct approach would be to solve the
>>>> problem. It seems that the local implicit instantiations need to be
>>>> performed earlier, or the validation of constexpr variables need to be
>>>> delayed.
>>>>
>>>> If someone can point me in the right direction it would be appreciated.
>>>>
>>>
>>> The implicit instantiations should be performed sooner. (FWIW, the same
>>> problem will affect member functions of local classes that have deduced
>>> return types.) Perhaps the easiest way to address this would be to perform
>>> all the local implicit instantiations discovered while instantiating a
>>> local class when we reach the end of that local class, instead of delaying
>>> them until the end of the surrounding function. We try to delay these
>>> instantiations as much as we can, to reduce the stack usage in recursive
>>> template instantiation, but that's probably not going to be significant
>>> here because there's not likely to be many layers of AST between the
>>> function and the class definition.
>>>
>>> One place to put this fix would be around
>>> SemaTemplateInstantiateDecl.cpp:1104:
>>>
>>>   // DR1484 clarifies that the members of a local class are instantiated
>>> as part
>>>   // of the instantiation of their enclosing entity.
>>>   if (D->isCompleteDefinition() && D->isLocalClass()) {
>>>     SemaRef.InstantiateClass(D->getLocation(), Record, D, TemplateArgs,
>>>                              TSK_ImplicitInstantiation,
>>>                              /*Complain=*/true);
>>>     SemaRef.InstantiateClassMembers(D->getLocation(), Record,
>>> TemplateArgs,
>>>                                     TSK_ImplicitInstantiation);
>>>   }
>>>
>>> You could save, perform, and restore the pending local implicit
>>> instantiations around these calls.
>>>
>>
>>
>
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