Wsrtjtyk

Consider the following program:

#include <iostream>



template<typename... Params_t>

constexpr int constexprValue(Params_t...) { return 5; }



int main()

{

    const bool flag = true;



    if constexpr(flag)

    {

        constexpr int value = constexprValue(1, 2, 3);

        std::cout << value << "n";

    }

}

This compiles and works fine. However, if flag is changed to false, then clang (Apple LLVM version 10.0.0 (clang-1000.10.44.4)) gives a compiler error:

error: constexpr variable 'value' must be initialized by a constant expression

undefined function 'constexprValue<int, int, int>' cannot be used in a constant expression

Is this a bug in clang?

Yes this is a bug it was fixed by this commmit to clang: [Sema] Discarded statment should be an evaluatable context. which has the following description:

The constexpr evaluator was erroring out because these templates weren't
defined. Despite being used in a discarded statement, we still need to constexpr
evaluate them, which means that we need to instantiate them. Fixes PR37585.

Differential revision: https://reviews.llvm.org/D48322

and includes the following test:

namespace PR37585 {

template <class T> struct S { static constexpr bool value = true; };

template <class T> constexpr bool f() { return true; }

template <class T> constexpr bool v = true;



void test() {

  if constexpr (true) {}

  else if constexpr (f<int>()) {}

  else if constexpr (S<int>::value) {}

  else if constexpr (v<int>) {}

}

}

If we try the test live with godbolt with an older clang version we obtain a very similar erroneous diagnostic that your example is seeing:

error: constexpr if condition is not a constant expression

else if constexpr (f<int>()) {}

                   ^~~~~~~~



note: undefined function 'f<int>' cannot be used in a constant expression

the fix originated from bug report: constexpr if condition is not a constant expression and std::is_same.

Yes, it seems to be a bug in the Apple version of clang, I'm able to compile the code with clang version 5 and greater as well as gcc version 7.1 and greater.

Matt Godbold has a great website for compiling snippets of code with a slew of different compilers.

Here is a link to your example in godbolt.