How to static assert a condition on a string literal inside of a constexpr function?

Question

In the following example main can static_assert if a string literal starts with 'v', but verify can not.

Why is it happening? Is there a way to allow verify to static_assert conditions on characters in a string literal?

#include <cstddef>

template <std::size_t N>
constexpr char get_first(const char (&str)[N])
{
    static_assert(N>1, "must be > 1");
    return str[0];
}

template <std::size_t N>
constexpr void verify(const char (&str)[N])
{
    static_assert(str[0] == 'v', "must start from v");   
}

int main()
{
    static_assert(get_first("value") == 'v', "first must be 'v'"); // succeeds
    verify("value"); // fails to compile
}

Compilation errors:

main.cpp: In instantiation of 'constexpr void verify(const char (&)[N]) [with long unsigned int N = 6]':
main.cpp:19:15:   required from here
main.cpp:13:9: error: non-constant condition for static assertion
         static_assert(str[0] == 'v', "must start from v");
         ^~~~~~~~~~~~~
main.cpp:13:9: error: 'str' is not a constant expression

Example.

Answer 1

I have another workaround for you. This will not use static_assert but is guaranteed to enforce the condition on compile time.

#include <type_traits>
template<bool b>
using enforce = std::bool_constant<b>;

template <std::size_t N>
constexpr int verify(const char (&str)[N])
{
    if(get_first(str) != 'v') {
        throw "must start from v";
    }
    return 0;
}

int main()
{
    using assertion = enforce<verify("value")>; // compiles
    using assertion = enforce<verify("fail")>; // fails to compile
    // or use it like
    constexpr auto assertion0 = verify("value"); // compiles
}

This uses that throwing is not valid in a constexpr context. The error you'll receive will look somethign like this:

26 : <source>:26:31: error: non-type template argument is not a constant expression
    using assertion = enforce<verify("fail")>; // fails to compile
                              ^~~~~~~~~~~~~~
15 : <source>:15:9: note: subexpression not valid in a constant expression
        throw "must start from v";
        ^
26 : <source>:26:31: note: in call to 'verify("fail")'
    using assertion = enforce<verify("fail")>; // fails to compile

We can enforce constexpr evaluation of verify by using it as a template argument. That's also the reason for declaring a non-void return type.