GCC bug or UB? Should this code compile?

Question

The code below compiles fine with clang, but does not compile with GCC (tried 4.1.2, 4.5.4 and 4.7.2):

template <typename T>
struct A
{
    struct B { };
};

template <typename T>
bool operator==(typename A<T>::B const& b, T const&  t);

enum { BAR };

template <typename T>
bool test()
{
    return 0 == BAR;
}

The error message from GCC 4.7.2 is:

a.cpp: In instantiation of ‘struct A<<anonymous enum> >’:
a.cpp:12:6:   required by substitution of ‘template<class T> bool operator==(const typename A<T>::B&, const T&) [with T = <anonymous enum>]’
a.cpp:19:17:   required from here
a.cpp:6:12: error: ‘<anonymous enum>’ is/uses anonymous type
a.cpp:6:12: error:   trying to instantiate ‘template<class T> struct A<T>::B’
a.cpp:6:12: error: ‘<anonymous enum>’ is/uses anonymous type
a.cpp:6:12: error:   trying to instantiate ‘template<class T> struct A<T>::B’

Is GCC correct in rejecting the code, or am I hitting its bug?

P.S. I've seen this error while trying to build one of opensource projects. I tried to make smallest possible example that reproduces it.

What function are you calling? Could you please post the entire a.cpp? — Anders Johansson, Dec 02 '12 at 06:55
@AndersJohansson This **is** entire a.cpp. I'm not calling any function. — iv_, Dec 02 '12 at 06:57
Looks like a g++ bug to me. Comeau online compiles the code as well. — Prasoon Saurav, Dec 02 '12 at 07:01
SFINAE should kick in and use the normal enum-to-int comparison. — GManNickG, Dec 02 '12 at 07:13
@iv_, maybe I'm missing something but there's no `main` and template functions are only instantiated when called, so if you're not calling any function, this a.cpp should definitely not compile, nor do anything. — Anders Johansson, Dec 02 '12 at 07:22
Can you submit the function call that causes the errors because it compiles fine in Ideone.com. — SChepurin, Dec 02 '12 at 07:23
@SChepurin: when I tried it, ideone.com throws the errors described. — Michael Burr, Dec 02 '12 at 08:43
@AndersJohansson: the code might not compile to a complete program, but it can still be compilable (use the `-c` option to just compile without linking). An empty file will compile successfully. — Michael Burr, Dec 02 '12 at 08:46
@GManNickG: I don't see any of these templates being instantiated - why would SFINAE kick in? — Michael Burr, Dec 02 '12 at 08:48
@Michael Burr "when I tried it, ideone.com throws the errors described."- And would you kindly submit the link to ideone.com with your test? — SChepurin, Dec 02 '12 at 08:56
@Michael Burr - What do you test? Does the order matter? Where is main()? — SChepurin, Dec 02 '12 at 09:28
`g++ a.cpp -c -std=c++0x` compiles fine. The bug seems to be fixed in the c++11 specific parts of g++. — Zeta, Dec 02 '12 at 10:18
i am not sure that this is a bug. in c++11, they simply changed the rules to be less strict. There are several rules that relate linkage-less entities with types that have linkage and restrict their relation in certain ways. Will read the relevant parts of the spec later.. — Johannes Schaub - litb, Dec 02 '12 at 11:18
@MichaelBurr: `0 == BAR` should be compiled during phase one, no instantiation needed. — GManNickG, Dec 02 '12 at 19:30
It's certainly not a bug, it's a change in C++11 and implemented as of gcc 4.4 if you specify the correct language version (but, only then, which is, again, correct). — Damon, Dec 04 '12 at 13:02

score 1 · Answer 1 · answered Dec 04 '12 at 12:40

1

It is not valid C++ according to the original standard:

14.3.1 of the standard says:

2 A local type, a type with no linkage, an unnamed type or a type compounded from any of these types shall not be used as a template argument for a template type-parameter. [Example: ...

However I believe this was restriction was removed by the latest C++11 standard. That may explain why some compilers accept it while others reject it.

answered Dec 04 '12 at 12:40

atomice

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1

As GManNickG pointed out in the comments, SFINAE. The failing overload should be silently eliminated. – MSalters Dec 04 '12 at 12:50
I confirm that `g++` is not compiling it until you ask for C++11 specs: `g++ -std=c++11 -o foo.o -c foo.c` – perror Dec 17 '12 at 12:44

GCC bug or UB? Should this code compile?

1 Answers1