readability-container-size-empty improperly converts `c.size() < 0` to `c.empty()`

[r-barnes]

Some objects can have a size() which returns negative values and also have an empty() method.

In this case, size() < 0 is not the same thing as empty().

Currently readability-container-size-empty converts x.size() < 0 to x.empty() - this has caused some real problems for us was an unexpected behaviour that isn't made explicit in either the documentation or the code itself.

[chrchr-github]

using size_type = int;
//using size_type = unsigned;
struct S {
    bool empty() const;
    size_type size() const;
};

void f(const S& s) {
    if (s.size() < 0) {}
}

https://godbolt.org/z/8f9TKjo8r
The warning appears also for an unsigned type, where replacement with empty() is (likewise) a bug.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Richard Barnes (r-barnes)

Some objects can have a `size()` which returns negative values and also have an `empty()` method.

In this case, size() < 0 is not the same thing as empty().

Currently readability-container-size-empty converts x.size() < 0 to x.empty() - this has caused some real problems for us was an unexpected behaviour that isn't made explicit in either the documentation or the code itself.