[Issue 22159] "==" causeses error for array of classes in safe method

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22159

--- Comment #3 from RazvanN <razvan.nitu1305 at gmail.com> ---
It's not a compiler bug. Here is where the error is emitted [1]. You can see
that the druntime call is constructed and then semantic is tried.
If there are any errors, the compiler simply assumes that the types are not
compatible. This was introduced by kinke in [2] so that the user
will not get errors pointing to druntime code. If in [2] we use
`expressionSemantic` instead of `trySemantic` we would get an error stating
that `core.object.opEquals` [3] is not @safe and cannot be @safe because it
calls `Object.opEquals`. In fact, just comparing 2 classes with no user-defined
opEquals - `assert (c == c)` - will issue an error in @safe code: "`@safe`
function `D main` cannot call `@system` function `object.opEquals`".

Therefore, the underlying issue is that `object.opEquals` is unsafe and cannot
be made @safe because then any class is constrained
to have a @safe opEquals. (I wonder if we can get around this by changing the
attribute inheritance rule - point 6 in [4])

[1] https://github.com/dlang/dmd/blob/master/src/dmd/expressionsem.d#L11416
[2]
https://github.com/dlang/dmd/pull/11212/files#diff-f0a120d764c7fab59bd3210b19fe685d2f106e4a1b9a33266804ae99dd3bbd4eR11135
[3] https://github.com/dlang/druntime/blob/master/src/object.d#L240
[4] https://dlang.org/spec/function.html#function-inheritance

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