Clang static parameter to std::fill causing linker to fail - c++

Question

Using Clang++ (v3.8.0), the following code fails to link due to sSomeValue being an undefined reference.
#include <iostream>
struct MyClass
{
static constexpr int sSomeSize = 3;
static constexpr int sSomeValue = 10;
};
int foo()
{
int someArray[MyClass::sSomeSize] = {};
std::fill(std::begin(someArray), std::end(someArray), MyClass::sSomeValue);
return someArray[0];
}
int main()
{
std::cout << foo() << std::endl;
}
More precisely:
clang++ -std=c++14 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
/tmp/main-c8de0c.o: In function `foo()':
main.cpp:(.text+0x2): undefined reference to `MyClass::sSomeValue'
/tmp/main-c8de0c.o: In function `main':
main.cpp:(.text+0x16): undefined reference to `MyClass::sSomeValue'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
However, changing the definition of foo to
int foo()
{
int someArray[MyClass::sSomeSize] = {MyClass::sSomeValue, MyClass::sSomeValue, MyClass::sSomeValue};
return someArray[0];
}
does not exhibit the same linker error, even though sSomeValue is still being used.
What is going on here? Is the compiler doing some optimization around the std::fill call that I may not be aware of?
Note that g++ -std=c++14 -O2 -Wall -pedantic -pthread main.cpp && ./a.out compiles, links, and outputs 10 as expected with v6.3.0.

Answer 1

The error is nothing to do with std::fill.
If you refer below documentation, it says: Reference variables can be declared constexpr (their initializers have to be reference constant expressions):
http://en.cppreference.com/w/cpp/language/constexpr
Slight modification of your code works fine. Just make the struct variables "const &" as said above.
#include <iostream>
struct MyClass
{
static constexpr int const& sSomeSize = 3;
static constexpr int const& sSomeValue = 10;
};
int foo()
{
int someArray[MyClass::sSomeSize] = {};
std::fill(std::begin(someArray), std::end(someArray), MyClass::sSomeValue);
return someArray[0];
}
int main()
{
std::cout << foo() << std::endl;
}
Also refer here for well explained article about constexpr and static
"Does static constexpr variable make sense?
Specially the last para in ticked answer.