std.mem.eql: make comparisons for zero-sized and non-sized types work

[wooster0]

Fixes #19929

[Rexicon226]

Maybe a bit of a strange idea, but what if the Undefined case returned, "undefined"?

std.mem.eql compares the contents of two slices for equality, and equating Undefined is impossible imo. Either "undefined" or false.

[wooster0]

Well if T is @TypeOf(undefined) then it doesn't compare what's in the slices in the first place and just looks at the length because it knows that the contents are all undefined, in this case all the same value anyway (in some sense at least). The same applies to @TypeOf(Null) as the type (no size available for type '@TypeOf(null)') so I've added a special case for that as well.
That, or I think std.mem.eql should simply not support comparing undefined. Maybe same for null.

A bit related to #15909

[Rexicon226]

in this case all the same value anyway (in some sense at least).

This is what I disagree with. There might be some existing precedent that supports the argument that untyped undefined is equal to another untyped undefined, but I don’t know if it.

I agree it probably would be best we just don’t support that.

[rohlem]

to make comparing enum{}s possible

I don't see a value in this particular function. enum{} is a non-instantiable type.
Control flow paths that encounter an instantiation of this type are by definition unreachable (= illegal behavior when reached). IMO coercions from undefined shouldn't be exempt from this.

[Rexicon226]

My 2c after thinking about this for a bit:

1. the Undefined one should totally be illegal, that is a really silly pattern that should never be written.
2. This is std.mem.eql. I think that the @sizeOf(T) == 0 check was OK to have because we are checking memory equality here. The solution to std.mem.eql not working with array of types #19929 is a std.meta function for slice checking if that's something we want supported. We are reaching outside the scope of the purpose of this function with this PR.

[wooster0]

I don't see a value in this particular function. enum{} is a non-instantiable type.

Me neither but the commit description is now outdated and enum{} is comparable without any special handling. Once enum{} becomes actually non-instantiable (it currently is instantiable) the two lines in test eql testing enum{} can simply be removed.

the Undefined one should totally be illegal, that is a really silly pattern that should never be written.

I'm going to wait for a decision from a core team member here before making any changes.

This is std.mem.eql. I think that the @sizeOf(T) == 0 check was OK to have because we are checking memory equality here.

Well in the case of type, comptime_int, and comptime_float it's comptime memory that's compared which is still memory.
But maybe this should instead be handled by std.meta.eql? Should std.mem.eql then error for type, comptime_int, and comptime_float to avoid confusing behavior?

Again, going to wait for a core team member's decision here.

[rohlem]

Once enum{} becomes actually non-instantiable (it currently is instantiable) the two lines in test eql testing enum{} can simply be removed.

I guess I view the tests as also living documentation of expected/intended behavior.
On any compiler change failing tests might be seen as indication that the change disagrees with the language usage within std.
That's why I'd personally exclude cases we already intend to drop support for at the time of writing.
However, I can see how that might be too abstract and subjective of a view, so it's presumably fine either way.