Does const reference have external linkage in C++?

Question

According to clause 3 of section 3.5 of C++ 1998 standard, a const reference has internal linkage.

A name having namespace scope (3.3.5) has internal linkage if it is the name of

• an object, reference, function or function template that is explicitly declared static or,

• an object or reference that is explicitly declared const and neither explicitly declared extern nor previously declared to have external linkage; or

• a data member of an anonymous union.

But why multiple definition conflict is generated when compiling the following code?

// a.cpp
const int& a = 1;

int main()
{
    return 0;
}

// b.cpp
const int& a = 1;

And then compile the code.

$ g++ a.cpp b.cpp
/tmp/ccb5Qi0M.o:(.bss+0x0): multiple definition of `a'
/tmp/ccD9vrzP.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status

If the const reference is changed to const, as follows

// a.cpp
const int a = 1;

int main()
{
    return 0;
}

// b.cpp
const int a = 1;

It is OK to compile.

Answer 1

The reference itself isn't const, just the object it refers to; so (arguably) this rule doesn't give the reference internal linkage.

It doesn't make sense for a reference to be declared const. The C++11 standard clarifies the wording:

a variable that is explicitly declared const or constexpr and neither explicitly declared extern nor previously declared to have external linkage

with no mention of the nonsensical concept of references declared const.