[Bug c++/106889] New: __builtin_strlen fails for some constexpr constructs

[m.cencora at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106889

            Bug ID: 106889
           Summary: __builtin_strlen fails for some constexpr constructs
           Product: gcc
           Version: 10.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.cencora at gmail dot com
  Target Milestone: ---

Compiling following program on any gcc version, with C++14 or newer standard
fails with
"'__builtin_strlen(...)' is not a constant expression" for some of the static
asserts.

It compiles successfuly on clang.


g++ -std=c++14

constexpr unsigned constexpr_strlen(const char* str)
{
#ifdef WORKAROUND
    if (!__builtin_constant_p(str))
    {
        unsigned i = 0;
        while (str[i] != '\0')
        {
           ++i;
        }
        return i;
    }
#endif
    return __builtin_strlen(str);
}

struct StaticString
{
    constexpr StaticString(const char (&src)[4])
        : buf{src[0], src[1], src[2], src[3]}
    {}

    constexpr const char* data() const
    {
        return &buf[0];
    }

    char buf[4];
};

constexpr StaticString createStaticString()
{
    return {"foo"};
}

constexpr const char *s1 = "foo";
constexpr const char s2[] = "foo";
constexpr StaticString s3{"foo"};
constexpr StaticString s4 = s2;
constexpr StaticString s5 = s4;
constexpr StaticString s6 = createStaticString();

void test()
{
    constexpr const char *s7 = "foo";
    constexpr const char s8[] = "foo";
    constexpr StaticString s9{"foo"};
    constexpr StaticString s10 = s8;
    constexpr StaticString s11 = s10;
    constexpr StaticString s12 = createStaticString();

    static constexpr const char *s13 = "foo";
    static constexpr const char s14[] = "foo";
    static constexpr StaticString s15{"foo"};
    static constexpr StaticString s16 = s14;
    static constexpr StaticString s17 = s6;
    static constexpr StaticString s18 = createStaticString();

    static_assert(constexpr_strlen(s1) == 3, "");
    static_assert(constexpr_strlen(s2) == 3, "");
    static_assert(constexpr_strlen(s3.data()) == 3, "");
    static_assert(constexpr_strlen(s4.data()) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s5.data()) == 3, ""); // ERROR 
    static_assert(constexpr_strlen(s6.data()) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s7) == 3, "");
    static_assert(constexpr_strlen(s8) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s9.data()) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s10.data()) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s11.data()) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s12.data()) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s13) == 3, "");
    static_assert(constexpr_strlen(s14) == 3, "");
    static_assert(constexpr_strlen(s15.data()) == 3, "");
    static_assert(constexpr_strlen(s16.data()) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s17.data()) == 3, ""); // ERROR
    static_assert(constexpr_strlen(s18.data()) == 3, ""); // ERROR
}