Error on tuple size with multi assign

[straight-shoota]

#11145 introduced a new compiler error when the target size in 1-to-n assignments don't match up.

That triggers when the assigned tuples has more elements than the number of targets:

a, b = {1, 2, 3} # Error: cannot assign Tuple(Int32, Int32, Int32) to 2 targets

When the assigned tuple has less elements than the number of targets, a similar error would be expected. But you get a different one:

a, b, c = {1, 2} # Error: index out of bounds for Tuple(Int32, Int32) (2 not in -2..1)

The expected error as per the introducing PR would be this:

a, b, c = {1, 2} # Error: cannot assign Tuple(Int32, Int32) to 3 targets

The reason is that the error specific to multi assign mismatch is triggered in CleanupTransformer, but before the compiler gets there, the expanded code already triggers the index error for accessing an out of range tuple element.

I'm not sure why this wasn't done properly. Maybe it was too complex? The initial PR already has a spec for this, but it's marked as pending.

This isn't a big issue. The important part is that there's a compiler error, it could just be more specific.

/cc @HertzDevil

[asterite]

@straight-shoota Could you clarify what's the expected behavior here? Both errors look fine to me.

[straight-shoota]

The expected error as per the introducing PR would be this:

a, b, c = {1, 2} # Error: cannot assign Tuple(Int32, Int32) to 3 targets