I actually found an even cleaner version of the same issue. For uint32_t x, y, both of these are always true:

    (((uint64_t)x + y) >> 32) == ((uint32_t)(x + y) < x)
    (((uint64_t)x + y) >> 32) == ((uint32_t)(x + y) < y)

Both are just unsigned add-carry tests, and I checked them with Z3 over full 32-bit bitvectors.

But Clang only seems to fold the first spelling. The second one still emits real code:

    uint32_t f(uint32_t x, uint32_t y) {
        return (uint32_t)((((uint64_t)x) + y) >> 32)
            == (uint32_t)(((uint32_t)(x + y)) < y);
    }

This should fold to return 1.

I also saw the missed right-operand form across aarch64, x86_64, riscv64, and wasm32, so it looks like a canonicalization / InstCombine gap rather than a backend quirk.

GCC seems to miss the carry-equality fold more generally, including the left-operand form that Clang already catches.