在另一个函数内转发声明`constexpr`函数 - 编译错误？

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 .