[cfe-dev] static constexpr member function of a local struct in a function template is instantiated too late
Richard Smith
richard at metafoo.co.uk
Mon Aug 25 12:56:56 PDT 2014
On Fri, Aug 22, 2014 at 11:39 AM, Michael Park <mcypark at gmail.com> wrote:
> 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?
>
It's fine. One minor thing: we prefer -p0 patches, and git produces -p1
patches by default. Use 'git diff --no-prefix'.
> 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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