A15-4-2: Ignore elided calls when calculating the exception graph

[lcartey]

Affected rules

• A15-4-2

Description

Copy elision allows the compiler to omit copy and move construction in certain circumstances. Some of these circumstances are considered "mandatory" - i.e. the compiler must omit the explicit construction - and in those circumstances copy and move constructors may never be called.

If you have move/copy constructors which are never expected to be called because they are always elided, you may decide to throw an exception from the copy/move constructor to enforce that it is only used in circumstances where the elision occurs. As our call graph does not currently reflect the mandatory elision of functions, this can cause false positives.

Example

// `foo` is only used in circumstances where it is elided, so this exception is never thrown in practice
constexpr foo(const foo&) : dummy() {
  throw some_bad_exception();
};

[lcartey]

See also:
#20
As it appears we do sometimes elide the copy/move constructors.

[lcartey]

Modify https://github.com/github/codeql-coding-standards/blob/main/cpp/common/src/codingstandards/cpp/exceptions/ExceptionFlow.qll#LL315C49-L315C97 to exclude calls to copy or move constructors we think are elided.

[lcartey]

Adding a reference to these related changes:
#641

This eliminates unused compiler generated elided functions. However, because the user in the above case explicitly added the elided function, I believe that's why we still add a call in this case. We should confirm by creating a test case.