[Bug c++/87381] clang 6.0 will compile this constexpr construct, but gcc 8.2.1 will not.

[eric-bugs at omnifarious dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87381

--- Comment #5 from eric-bugs at omnifarious dot org ---
Here is the problem, reduced to the simplest expression I could make:

-----
template <int x>
struct test_template {
    static int size() { return x; }
};

constexpr int ce_strlen(char const *s)
{
    int i = 0;
    while (s[i]) ++i;
    return i;
}

int joe()
{
    constexpr int plen = ce_strlen(__PRETTY_FUNCTION__); // This works
    test_template<plen> a; // This declaration is valid.
    test_template<ce_strlen(__PRETTY_FUNCTION__)> b; // But this doesn't work?!
    return a.size() + b.size();
}
------

Either the declaration/initialization of both plen and b should fail, or they
should both succeed. It makes no sense for one to work and the other to not.