Why do these two pieces of code using constexpr, __PRETTY_FUNCTION__ and char * have different results?

Question

I have this code where if you comment out the line commented "But this doesn't work?!" it compiles just fine, but if you don't, the compiler generates an error.

At least, gcc 8.2 generates an error.

But, they seem identical to me. What's the problem? Is this legal code at all?

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();
}

I ran into this while trying to come up with a way to create profile tags for an intrusive profiling system at compile time. I succeeded, but my final code does not involve using ce_strlen.

Answer 1

Indeed this is a bug in GCC as discussed in the comments, but I thought I'd throw in some additional insight as to the nature of this bug. In the GCC NEWS file there is this line:

• __FUNCTION__ and __PRETTY_FUNCTION__ are now treated as variables by the parser; previously they were treated as string constants. So code like printf (__FUNCTION__ ": foo") must be rewritten to printf ("%s: foo", __FUNCTION__). This is necessary for templates.

But __PRETTY_FUNCTION__ isn't really a variable, it's a special case treated in the parser as we see in constexpr.c:

    case DECL_EXPR:
      {
    tree decl = DECL_EXPR_DECL (body);
    if (TREE_CODE (decl) == USING_DECL
        /* Accept __func__, __FUNCTION__, and __PRETTY_FUNCTION__.  */
        || DECL_ARTIFICIAL (decl))
      return NULL_TREE;
    return error_mark_node;
}

If it really was a variable, we'd expect it to pass the same test cases as these:

constexpr const char* s2 = "TEST";
constexpr const char* s3 = s2;
test_template<ce_strlen("TEST")> c;
test_template<ce_strlen(s2)> d;
test_template<ce_strlen(s3)> e;