为什么此模板constexpr函数不能在gcc上编译，但是在clang上能很好地工作？

Question

As you can see here http://melpon.org/wandbox/permlink/vJSyO14mkbH0MQRq this doesn't compile on gcc with the error:

prog.cc: In instantiation of 'constexpr B convert(A) [with A = unsigned char; B = short unsigned int]':
prog.cc:16:52:   required from here
prog.cc:12:1: error: body of constexpr function 'constexpr B convert(A) [with A = unsigned char; B = short unsigned int]' not a return-statement

The code:

#include <stdint.h>
#include <limits>
#include <iostream>

template< typename A, typename B >
constexpr B convert( A a )
{
    auto aMax = std::numeric_limits< A >::max();
    auto bMax = std::numeric_limits< B >::max();

    return a * ( bMax / aMax );
}

int main()
{
    std::cout << convert< uint8_t, uint16_t >( 128 ) << std::endl;
    return 0;
}

Answer 1

This code requires a C++14 feature called "Relaxing constraints on constexpr-functions". It is supported in Clang since version 3.4, but GCC didn't implement it yet and subsequently complains about your function template.

There is no need for C++14 though, just rewrite it as

template <typename B, typename A> //! Note that I reordered the parameters
constexpr B convert( A a )
{
    return a * (std::numeric_limits< B >::max() / std::numeric_limits< A >::max());
}

You can also use alias declarations.

template <typename T, T v>
using iconst = std::integral_constant<T, v>;

template <typename B, typename A>
constexpr B convert( A a )
{
    using aMax = iconst<A, std::numeric_limits< A >::max()>;
    using bMax = iconst<B, std::numeric_limits< B >::max()>;
    return a * (bMax::value / aMax::value);
}

Demo