Updated specification for Std.is / safe cast

[ncannasse]

Given there was a disagreement over the specification of Std.is / safe cast, I'm using my dictatorship power to move forward for Haxe 4.0:

Std.is(null,T) should be false : null value has no runtime type information attached so while it's a valid value for a variable of type T it's not an object instance of T either, which is what Std.is is checking.

cast(null,T) should be null (not throw), because in that case we convert the value null to type T and it's valid value for this type. Also both (null : T) and (cast null : T) works. That's the way type casting works in C#, Java, and many other languages.

Requirements:

• write corresponding unit tests
• ensure platforms compliance
• document platform changing behaviors for release notes

Additional checks:

• null can only be caught by catch( e : Dynamic )
• cast(null,Int|Float|Bool) : on static platforms, return the default value (same as unsafe cast). on dynamic platorms, returns null.
• Std.is(null,Int|Float|Bool|Dynamic): should be false on all platforms as well

[Simn]

The canonical implementation of cast(v, T) is if (Std.is(v, T)) cast v else throw "Class cast error", so this makes no sense to me.

[ncannasse]

The new one will be if (Std.is(v, T) || v == null ) cast v else throw "Class cast error"

[ncannasse]

I don't want another discussion topic, thanks

[Simn]

Closing in favor of #7532.