[cfe-dev] unnecessary implicit instantiation?

[Troy Johnson via cfe-dev]

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<mailto: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<mailto:cfe-dev-bounces at lists.llvm.org>> on behalf of Troy Johnson via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
Sent: Wednesday, September 18, 2019 4:26 PM
To: cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org> <cfe-dev at lists.llvm.org<mailto: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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