[Bug sanitizer/71962] error: ‘((& x) != 0u)’ is not a constant expression with f sanitize=undefined

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71962

Bjorn Pagen <hello at bjornpagen dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hello at bjornpagen dot com

--- Comment #20 from Bjorn Pagen <hello at bjornpagen dot com> ---
This bug also affects C++26 reflection and consteval code in GCC 16.1.0.

This standalone testcase reproduces the std::string(ptr, size) failure over
vague-linkage storage:

#include <string>

inline constexpr char text[] = "x";

consteval auto check() -> bool {
  auto value = std::string{text, 1};
  return value == "x";
}

static_assert(check());

Command:

g++-16 -std=c++26 -fsanitize=undefined -c repro.cc

GCC rejects the static assertion. The final diagnostic is:

bits/basic_string.h:713:17: error: '(((const char*)(& text)) == 0)' is not a
constant expression

The same command succeeds without -fsanitize=undefined.

Reflection APIs can expose text through compiler-generated storage with the
same linkage property. This bug therefore blocks required constant evaluation
in reflection code.

Attachment 65251, added on 2026-08-05, addresses the same separation between
constant evaluation and null-check optimization. This testcase can extend its
regression coverage.