Forward-declaration of a `constexpr` function inside another function -- Compiler bug?

Question

While producing a MCVE for this problem I stumbled upon, I've found the following discrepancy between compilers:

Consider the following code :

// constexpr int f(); // 1

constexpr int g() {
    constexpr int f(); // 2
    return f();
}

constexpr int f() {
    return 42;
}

int main() {
    constexpr int i = g();
    return i;
}

This code compiles on Clang 3.8.0, but fails on GCC 6.1.0 with:

error: 'constexpr int f()' used before its definition

Commenting out // 2 and uncommenting // 1 works on both compilers.

Interestingly, moving f's definition in place of // 1 compiles, but triggers a warning at // 2:

warning: inline function 'constexpr int f()' used but never defined

Which compiler is right ?

Answer 1

Replacing the constexpr functions with inline functions retains the exact same problem (it is okay with the global declaration 1, but not with the function-scope declaration 2.) Since constexpr implies inline this seems like the cause.

In this case, with declaration 2, GCC complains: warning: 'inline' specifier invalid for function 'f' declared out of global scope and warning: inline function 'int f()' used but never defined. It fails to link ("undefined reference to 'f()'").

So it looks like it gives up on inlining, puts in a call, but doesn't bother emitting code for f() because all uses are inlined (?), so the link fails.

and Clang complains: error: inline declaration of 'f' not allowed in block scope

Since constexpr implies inline, it seems that this rule that inline declarations are not allowed in block scope should also apply to constexpr, and so GCC is correct. But the standard does not seem to come out and say this. In the draft I examined, the rule about inline is in §7.1.2 [dcl.fct.spec], part 3: "The inline specifier shall not appear on a block scope function declaration", but nothing similar appears about constexpr.