[Bug c++/112296] New: __builtin_constant_p doesn't propagate through member functions

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112296

            Bug ID: 112296
           Summary: __builtin_constant_p doesn't propagate through member
                    functions
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Here's a short example:

using size_t = decltype(sizeof(0));

struct Span {
    int const* ptr;
    size_t len;

    inline constexpr auto size() const noexcept -> size_t { return len; }
};

inline int direct(Span span) {
    return __builtin_constant_p(span.size());
}

inline int indirect(Span span) {
    size_t s = span.size();
    return __builtin_constant_p(s);
}

int call_direct(int const* p) {
    return direct({.ptr=p, .len=8});
}

int call_indirect(int const* p) {
    return indirect({.ptr=p, .len=8});
}



The functions direct() and indirect() do the same thing - try to see if span's
size is constant, with direct checking size() directly and indirect first
caching it to a local variable and checking that variable. In both cases, the
size is constant - but on -O3 call_direct() returns 0 while call_indirect()
returns 1. That is, __builtin_constant_p tells me the size is constant - but
only if I put it into a variable first. 

Checking __builtin_constant_p(span.len) also returns 1, but in the real code
the member variable is private and my only access to it is via the size()
function.