extern array as non-type template argument with clang c++1z

Question

template<int const * pci> struct X {};
extern int const ai[];
X<ai> xi;
int const ai[] = {0,1,2,3};

If I try to compile this code with "clang++ -std=c++1z" it result in error:

test.cpp:4:3: error: non-type template argument refers to subobject '&ai'

But it isn't subobject.

http://en.cppreference.com/w/cpp/language/template_parameters doesn't list any suitable limitation for extern arrays in '(since C++17)' section for non-type arguments.

Such code works fine with -std=c++14. And GCC also compiles it without errors in c++1z mode: https://godbolt.org/g/K9wZ4g

Is it a clang bug? Or is this code wrong?

"But it isn't subobject." Yes, it is a subobject. An array in C++ can decay into a pointer to the first subobject element in the array. Now, that's not the say that the compiler should fail for that. But it is getting a pointer to a subobject. — Nicol Bolas
– Nicol Bolas, Commented Jul 1, 2016 at 13:52
@NicolBolas The fun part about this is that [temp.arg.nontype]/1 bans pointers to subobjects, and then the next paragraph has an example that matches the OP's. — T.C.
– T.C., Commented Jul 1, 2016 at 18:55
Now I'm totally confused. I've reread what standard means by subobject and yes, array element is a subobject. Looks like there is nothing in the standard which allows using arrays as pointer non-type template arguments. Not only extern but any arrays. — Ryhor Spivak
– Ryhor Spivak, Commented Jul 1, 2016 at 19:50

Shuo Chen · Accepted Answer · 2017-09-07 00:17:45Z

3

Yes, it's a clang bug, confirmed by clang developer and has been fixed in trunk (r311970). http://lists.llvm.org/pipermail/cfe-dev/2017-August/055249.html

Meanwhile, to work around it, you need to write the length of array in the declaration explicitly.

extern int const ai[4];

answered Sep 7, 2017 at 0:17

Shuo Chen

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