[Bug c++/90691] New: [GCC9 regression] -Wsign-compare false-positive with constant

[lebedev.ri at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90691

            Bug ID: 90691
           Summary: [GCC9 regression] -Wsign-compare false-positive with
                    constant
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lebedev.ri at gmail dot com
  Target Milestone: ---

Manually reduced:

#include <cstdint>

struct S {
  int a;

  constexpr S();
  explicit constexpr S(int a_) : a(a_) {}
};

constexpr S b = S(12);

template <const S& e>
bool c(unsigned int d) {
  return d >= e.a;
}

bool test(unsigned int d);
bool test(unsigned int d) {
    return c<b>(d);
}

GCC diagnoses:
<source>: In instantiation of 'bool c(unsigned int) [with const S& e = b]':
<source>:19:18:   required from here
<source>:14:12: warning: comparison of integer expressions of different
signedness: 'unsigned int' and 'const int' [-Wsign-compare]
   14 |   return d >= e.a;
      |          ~~^~~~~~
Compiler returned: 0

clang doesn't.
https://godbolt.org/z/7xrWzz


But there can't ever be any issues here, the signed 'int a' is '12', which is
known-positive.
Indeed, if the constant is negative, clang warns:
https://godbolt.org/z/yQBXB8