[vim9script] On extending interfaces

Vim 9 script allows the following definitions, in which I2 extends I1 by
"implementing" it:

interface I1
def Foo()
endinterface

interface I2 implements I1
def Foo()
def Bar()
endinterface

The above compiles: I don't know whether it is intentional, but it is
pretty cool! Or, it would be if the following code worked:

def Echo(obj: I1)
obj.Foo()
enddef

class C2 implements I2
def Foo()
echo 'C2'
enddef

def Bar()
enddef
endclass

const c2 = C2.new()
Echo(c2) # ERROR

This results in:

type mismatch, expected object<I1> but got object<C2>

But C2 does conform to I1! To fix the error, it is necessary to declare
all the interfaces implemented by C2, that is:

class C2 implements I1, I2
# etc.

I will mention two other minor issues:

- I2 must declare Foo() again: it would be nice if that definition could
be inferred.
- "implements" is not a very accurate description: "extends" would make
more sense, intuitively.

In summary, what I am asking is whether Vim could (or should) support
this syntax:

interface I1
def Foo()
endinterface

interface I2 extends I1
def Bar()
endinterface

with the following implications:

1. any class implementing I2 must implement both Foo() and Bar().
2. any object of a class implementing I2 may be used wherever an object
with type I1 is expected.

Thanks,
Life.

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