Update comments and add support for IDim+1+2 again (in foast_to_itir)

src/gt4py/next/ffront/foast_passes/type_deduction.py

@@ -629,11 +629,12 @@ def _deduce_binop_type(
             # this relies on the convention that for each dimension we have an offset provider
             # with the same name as the dimension
             return ts.OffsetType(source=left.type.dim, target=(left.type.dim,))
+        # e.g. `IDim+1+2`: this is currently only implemented for embedded
         if (
             isinstance(left.type, ts.OffsetType)
             and isinstance(right.type, ts.ScalarType)
             and type_info.is_integral(right.type)
-        ): #TODO: add comment
+        ):
             return ts.OffsetType(source=left.type.source, target=(left.type.source,))
 
         logical_ops = {

src/gt4py/next/ffront/foast_to_itir.py

@@ -309,6 +309,22 @@ def _visit_shift(self, node: foast.Call, **kwargs: Any) -> itir.Expr:
                     if node.args[i].op == dialect_ast_enums.BinaryOperator.SUB:  # type: ignore[attr-defined] # ensured by pattern
                         offset_index *= -1
                     shift_offsets.append(im.shift(f"{dimension}off", offset_index))
+                case foast.BinOp(
+                    op=dialect_ast_enums.BinaryOperator.ADD
+                       | dialect_ast_enums.BinaryOperator.SUB,
+                    left=foast.BinOp(
+                        op=dialect_ast_enums.BinaryOperator.ADD
+                           | dialect_ast_enums.BinaryOperator.SUB,
+                        left=foast.Name(id=dimension),
+                        right=foast.Constant(value=offset_index_left),
+                    ),
+                    right=foast.Constant(value=offset_index),
+                ):
+                    if node.args[i].op == dialect_ast_enums.BinaryOperator.SUB:
+                        offset_index *= -1
+                    shift_offsets.append(
+                        im.shift(f"{dimension}off", offset_index_left + offset_index)
+                    )
                 case foast.Name(id=offset_name):
                     assert len(node.args) == 1
                     return im.lifted_neighbors(