[next] Bug in embedded implementation of premap

[egparedes]

This issue has been originally discovered by @nfarabullini . I'm only reporting it there since she is on vacation.

In a new Icon4py PR (https://github.com/C2SM/icon4py/pull/472/files) the field-vire embedded implementation of a premap operation crashes for unclear reasons since it was working previously for iterator embedded, therefore this is most likely a bug.

This is the definition of the operator crashing:

C2E2CODim = Dimension("C2E2CO", DimensionKind.LOCAL)
C2E2CO = FieldOffset("C2E2CO", source=CellDim, target=(CellDim, C2E2CODim))

@field_operator
def _compute_weighted_cell_neighbor_sum(
    field: Field[[CellDim, KDim], wpfloat],
    c_bln_avg: Field[[CellDim, C2E2CODim], wpfloat],
) -> Field[[CellDim, KDim], wpfloat]:
    field_avg = neighbor_sum(field(C2E2CO) * c_bln_avg, axis=C2E2CODim)
    return field_avg

[havogt]

Reading again the description of the implementation, it seems we overlooked the case X2X for a Field[Dims[X,Y],...]. In this case the domain of the connectivity is partially (but not fully contained in the domain of the field).

[havogt]

This line

gt4py/src/gt4py/next/embedded/nd_array_field.py

Line 497 in 9d1e4e9

if self.domain.dim_index(self.codomain) is None

is problematic.
It makes the above case a ALTER_STRUCT only. If we would implement the behavior that is described, we should raise an exception here. But that wouldn't solve the issue. Let's re-discuss how the above case fits in the categorization of connectivities.
Side-note: If I force ALTER_STRUCT | ALTER_DIMS on the connectivity the example works.