[llvm-bugs] [Bug 42000] New: __builtin_sub_overflow reports invalid overflow in constexpr context

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42000

            Bug ID: 42000
           Summary: __builtin_sub_overflow reports invalid overflow in
                    constexpr context
           Product: clang
           Version: trunk
          Hardware: PC
                OS: FreeBSD
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: m at mikaelsimonsson.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

This code fails to compile:

#include <cstdint>

constexpr bool overflows(uint8_t a, int b)
{
    uint8_t diff = 0;
    return __builtin_sub_overflow(a, b, &diff);
}

int main()
{
    static_assert(!overflows(255, 100));
    return 0;
}

11:5: error: static_assert failed due to requirement '!overflows(255, 100)'
    static_assert(!overflows(255, 100));
    ^             ~~~~~~~~~~~~~~~~~~~~
1 error generated.


But this code compiles and runs without triggering the assert:

#include <cassert>
#include <cstdint>

inline bool overflows(uint8_t a, int b)
{
    uint8_t diff = 0;
    return __builtin_sub_overflow(a, b, &diff);
}

int main()
{
    assert(!overflows(255, 100));
    return 0;
}


Tested with godbolt.org: clang version 9.0.0 (trunk 361583)

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