Constexpr in facade.hpp

[minaevmike]

Constexpr in facade.hpp

[3Hren]

It can. It's GCC 5.3 bug.

[minaevmike]

i use gcc 5.4

[3Hren]

Well, that means that both GCC 5.3 and 5.4 are shit :)

> cat prog.cc

#include <stdexcept>

constexpr int f(int n) {
    if (n <= 0) throw std::runtime_error("");
    return 1;
}

int main() {
    constexpr auto val = f(42);   
}

Using GCC 5.3:

> g++ prog.cc -Wall -Wextra -std=gnu++1z

prog.cc: In function 'constexpr int f(int)':
prog.cc:6:1: error: expression '<throw-expression>' is not a constant-expression
 }
 ^
prog.cc: In function 'int main()':
prog.cc:9:27: error: 'constexpr int f(int)' called in a constant expression
     constexpr auto val = f(42);   
                           ^
prog.cc:9:20: warning: unused variable 'val' [-Wunused-variable]
     constexpr auto val = f(42);   
                    ^

Using both clang and GCC 6.0 (and trunk):

> g++ prog.cc -Wall -Wextra -std=gnu++1z

prog.cc: In function 'int main()':
prog.cc:9:20: warning: unused variable 'val' [-Wunused-variable]
     constexpr auto val = f(42);
                    ^~~

You may send a PR, but I don't see the correct way to resolve this issue.

There are compiler-defined macro variable that are used to check whether relaxed constexpr are supported by the compiler. Here they are:

#if defined(__cpp_constexpr) && __cpp_constexpr >= 201304

GCC 5.3 defines them, but the implementation is terrible, leading to such kind of errors.

See http://stackoverflow.com/questions/34280729/throw-in-constexpr-function also.

[3Hren]

I can suppress all constexpr magic for GCC 5.*

Please create a test, which compiles on trunk compilers, but fails to compile on GCC 5.*

[minaevmike]

i am not sure is it a good fix or not, but locally i change

#if defined(__cpp_constexpr) && __cpp_constexpr >= 201304

to

#if (__GNUC__ >= 6 || defined(__clang__)) && defined(__cpp_constexpr) && __cpp_constexpr >= 201304

[3Hren]

It's totally ok. Historically it's impossible to build with GCC without hacking with #ifdef.