undefined reference to class static constexpr struct, g++ vs clang

Question

This is my code, a.cpp

struct int2
{
    int x, y;
};
struct Foo{
    static constexpr int bar1 = 1;
    static constexpr int2 bar2 = {1, 2};
};
int foo1(){
    return Foo::bar1; // this is ok for both clang++ and g++
}
int2 foo2(){
    return Foo::bar2; // undefined reference to `Foo::bar2' in clang++
}
int main(){ std::cout << foo2().x << std::endl; return 0; }

use clang to compile, clang++ -std=c++11 a.cpp

/tmp/a-0dba90.o: In function `foo2()':
a.cpp:(.text+0x18): undefined reference to `Foo::bar2'
clang-7: error: linker command failed with exit code 1 (use -v to see 
invocation)

g++ -std=c++11 a.cpp emits no error.

My question is,

1. who is right on the above code? clang or g++?
2. why bar2 is wrong while bar1 is correct in clang?

compiler version: g++ 5.4.0 and clang 7.0.0

UPDATE: The question is marked as a duplicate of another question, but it is not. I know I could add explicitly definition outside class to get it pass for clang. This question is about why all the difference between g++&clang.

Answer 1

You seem to assume that if one compiler is right, the other one must be wrong. The program either contains an error (and then the compiler that accepts it is wrong) or it doesn't (and then the compiler that rejects it is wrong). This in turn relies on an implicit assumption that the error in question, namely, missing definition of an ODR-used entity, is a diagnosable error. Unfortunately it isn't. The standard explicitly states that:

[basic.def.odr/10] Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement; no diagnostic required.

As problematic and unwanted this provision of the standard is, it's there. Your program has undefined behaviour because of a missing definition, and an implementation is not required to diagnose it. So both compilers are technically correct at any optimisation level.

In C++17 with mandatory copy elision the program no longer contains any ODR-use of the variable in question constexpr static data members are implicitly inline, no separate deinition needed (thanks Oliv).