[BUG] Fail to interpret x:y:z as Slice in __getitem__ and in Variant

[forFudan]

Bug description

I am working on indexing and slicing by means of the __getitem__ method of structs. I noticed that when the arguments are Variant[Slice, AnyType), the syntax x[Slice(0,10,1)] works but x[0:10:1] fails.

It seems that x:y:z cannot be interpretted by compiler as Slice in square brackets.

Steps to reproduce

Here is an example that prints either an Int or the start of a Slice.

The error message is also provided.

We can see that Slice(5, 2, 1) works but foo[5:2:1] fails.

from utils import Variant


fn main():
    var foo = Foo(List[Int](0, 1, 2, 3, 4, 5, 6, 7))
    foo[5]  # works
    foo[Slice(5, 2, 1)]  # works
    foo[5:2:1]  # fails!
# error: invalid call to '__getitem__': index  cannot be converted from initializer list to 'Variant[Int, Slice]'
#     foo[5:2:1]
    ~~~^~~~~~~


struct Foo:
    var buf: List[Int]

    fn __init__(out self, data: List[Int]):
        self.buf = data

    fn __getitem__(self, i: Variant[Int, Slice]):
        if i.isa[Int]():
            print(self.buf[i._get_ptr[Int]()[0]])
        else:
            var s = i._get_ptr[Slice]()[0]
            print(self.buf[s.start.or_else(1)])

Error message:

error: invalid call to '__getitem__': index cannot be converted from initializer list to 'Variant[Int, Slice]'
    foo[5:2:1]
    ~~~^~~~~~~
note: try resolving the overloaded function first
    foo[5:2:1]
         ^
note: function declared here
    fn __getitem__(self, i: Variant[Int, Slice]):
       ^

System information

- What OS did you do install Mojo on?
MacOS 15.2
- Provide version information for Mojo by pasting the output of `mojo -v`
mojo 24.6.0 (4487cd6e)
- Provide Magic CLI version by pasting the output of `magic -V` or `magic --version` 
magic 0.5.1 - (based on pixi 0.37.0)
- Optionally, provide more information with `magic info`.

     Magic version: 0.5.1
System
------------
      Pixi version: 0.37.0
          Platform: osx-arm64
  Virtual packages: __unix=0=0
                  : __osx=15.2=0
                  : __archspec=1=m2
         Cache dir: /Users/ZHU/Library/Caches/rattler/cache
      Auth storage: /Users/ZHU/.rattler/credentials.json
  Config locations: No config files found

Global
------------
           Bin dir: /Users/ZHU/.modular/bin
   Environment dir: /Users/ZHU/.modular/envs
      Manifest dir: /Users/ZHU/.modular/manifests/pixi-global.toml

Project
------------
              Name: NuMojo
           Version: 0.4.0
     Manifest file: /Users/ZHU/Programs/NuMojo/mojoproject.toml
      Last updated: 22-12-2024 19:54:34

Environments
------------
       Environment: default
          Features: default
          Channels: conda-forge, https://conda.modular.com/max
  Dependency count: 4
      Dependencies: max, python, numpy, scipy
  Target platforms: osx-arm64, linux-64
             Tasks: p, package, final, test, t, f, format