C++ standard: do namespace-scoped constexpr variables have internal linkage?

Question

Imagine we have a header foo.h containing the following:

#ifndef FOO_H_
#define FOO_H_

namespace foo {
constexpr std::string_view kSomeString = "blah";
}

#endif  // FOO_H_

Is foo::kSomeString guaranteed to have internal linkage in any translation unit that includes foo.h? Does this vary between C++11 and C++17?

In the draft standard [basic.link]/3 says

A name having namespace scope has internal linkage if it is the name of [...] a non-inline variable of non-volatile const-qualified type that is neither explicitly declared extern nor previously declared to have external linkage [...]

But I don't know if constexpr counts as "const-qualified". Does the standard say so somewhere?

Assuming this is guaranteed to have internal linkage, it seems like there can be no problem with the ODR for this usage, right? (In contrast to what it says in this answer.)

Answer 1

Yes, constexpr on an object declaration means that the object is const. See [dcl.constexpr]/9. And yes, that means that kSomeString in your example has internal linkage.

The species of ODR violation we are talking about here is not the definition of kSomeString itself, but other definitions that attempt to use it. And there's a problem precisely because of the internal linkage. Consider:

void f(const std::string_view &);

inline void g() { 
    f(foo::kSomeString); 
}

This is an ODR violation if included in multiple translation units, essentially because the definition of g in each translation unit references a different object.