[cfe-dev] unnecessary implicit instantiation?

[David Blaikie via cfe-dev]

I don't think it's instantiating the static member - it's only type
checking. (in the same way that "template<typename T> struct foo { my_int
x; };" would be invalid if "my_int" wasn't previously defined)


Ah, here we go: from C++17, 17.6 [temp.res]p8:
"Knowing which names are type names allows the syntax of every template to
be checked. The program is ill-formed, no diagnostic required, if:"
"(8.3) - a hypothetical instantiation of a template immediately following
its definition would be ill-formed due to a construct that does not depend
on a template parameter"

Sounds pretty convincing to me, at least.


On Mon, Sep 23, 2019 at 10:52 AM Troy Johnson <troyj at cray.com> wrote:

> Thank you.  That would appear to contradict C++11 14.7.1 paragraph 8 which
> said "The implicit instantiation of a class template does not cause any
> static data members of that class to be implicitly instantiated."  This
> paragraph seems to have expanded into C++17 17.8.1 paragraph 10, but still
> includes static data members.
>
> Could you explain how this example does not violate that language?
>
> -Troy
> ------------------------------
> *From:* David Blaikie <dblaikie at gmail.com>
> *Sent:* Monday, September 23, 2019 11:23 AM
> *To:* Troy Johnson <troyj at cray.com>
> *Cc:* cfe-dev at lists.llvm.org <cfe-dev at lists.llvm.org>
> *Subject:* Re: [cfe-dev] unnecessary implicit instantiation?
>
> I believe the rules are that since 'M' is not dependent on any template
> parameters, it can be checked without deferring until the point of
> instantiation. (& that it isn't /required/ to be checked - it's up to the
> compiler, so both GCC and Clang are correct here)
>
> On Mon, Sep 23, 2019 at 8:07 AM Troy Johnson via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> If there's a different email list for C++ language questions like this,
> please let me know, since I'm trying establish which compiler's behavior is
> correct before I try to change anything in Clang.  The standard is a little
> fuzzy on this matter because it says that the instantiation occurs as
> needed, and one could argue that one does not need to know the size of a
> static data member in order to use the class type as part of another
> definition.  Clang seems to want the whole definition of the class,
> including static data, to be fully defined before using it.
>
> -Troy
> ------------------------------
> *From:* cfe-dev <cfe-dev-bounces at lists.llvm.org> on behalf of Troy
> Johnson via cfe-dev <cfe-dev at lists.llvm.org>
> *Sent:* Wednesday, September 18, 2019 4:26 PM
> *To:* cfe-dev at lists.llvm.org <cfe-dev at lists.llvm.org>
> *Subject:* [cfe-dev] unnecessary implicit instantiation?
>
> Clang issues an error for the following code, whereas the GNU compiler
> does not.  Based on the standard, I think that an implicit instantiation
> should not occur because the compiler is not yet allocating an object, and
> that would mean that Clang is incorrectly issuing an error and GNU is
> correct to not issue any error.
>
> Is my understanding correct?
>
> > cat t.cpp
> template <typename T> class A;
>
> template <typename T>
> struct B {
>   static A<int> M;
> };
>
> template <typename T> A<int> B<T>::M;
> > g++ -c t.cpp
> > clang -c t.cpp
> t.cpp:8:36: error: implicit instantiation of undefined template 'A<int>'
> template <typename T> A<int> B<T>::M;
>                                    ^
> t.cpp:1:29: note: template is declared here
> template <typename T> class A;
>                             ^
> 1 warning and 1 error generated.
>
>
> Thanks,
> Troy
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>
>
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