constexpr expression and variable lifetime, an example where g++ and clang disagree

Question

Consider the simple C++11 code:

template
struct Foo {};

template 
constexpr int size(const Foo&) { return N; }

template          


        

Answer 1

My interpretation is that clang++ is right and g++ is too permissive.

We can find a close example ([expr.const] section, page 126) in the standard https://isocpp.org/std/the-standard (draft can be downloaded, attention big PDF! ).

constexpr int g(int k) { 
    constexpr int x = incr(k); 

    return x; 
}

where it is explained that:

error: incr(k) is not a core constant expression because lifetime of k began outside the expression incr(k)

This is exactly what is happening in the use_size() function with the foo argument, even if the size() function only use the N template parameter.

template 
constexpr int size(const Foo&) { return N; }

template 
void use_size(const Foo& foo) { constexpr int n = size(foo); }