[Bug other/94741] New: stringop-truncation is triggered or not depending on surrounding members

[danregister at poczta dot fm]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94741

            Bug ID: 94741
           Summary: stringop-truncation is triggered or not depending on
                    surrounding members
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: danregister at poczta dot fm
  Target Milestone: ---

The example code compiled with "-Wall -Werror -O2" gives:

main.cpp: In constructor ‘S3::S3(const char*)’:
main.cpp:26:22: error: 'char* __builtin_strncpy(char*, const char*, long
unsigned int)' specified bound 5 equals destination size
[-Werror=stringop-truncation]

   26 |     __builtin_strncpy(a_, a, kMaxLen);

It complains only about S3 while it should complain about all of them.

The example code:

constexpr auto kMaxLen = 5;

struct S1 {             // just array
    S1(const char* a);
    char a_[kMaxLen];
};
S1::S1(const char* a) {
    __builtin_strncpy(a_, a, kMaxLen);
}

struct S2 {             // int member before array
    S2(const char* a);
    int n_;
    char a_[kMaxLen];
};
S2::S2(const char* a) {
    __builtin_strncpy(a_, a, kMaxLen);
}

struct S3 {             // int member after array
    S3(const char* a);
    char a_[kMaxLen];
    int n_;
};
S3::S3(const char* a)  {
    __builtin_strncpy(a_, a, kMaxLen);
}