LiteralEnum,

    union_cast_method_impl!(as_literal_struct, InferenceRuleLiteralStruct, InferenceRule::LiteralStruct);

    union_cast_method_impl!(as_literal_union, InferenceRuleLiteralUnion, InferenceRule::LiteralUnion);

    union_cast_method_impl!(as_literal_array, InferenceRuleLiteralArray, InferenceRule::LiteralArray);

    union_cast_method_impl!(as_literal_tuple, InferenceRuleLiteralTuple, InferenceRule::LiteralTuple);

        poly_progress_section.forget();

            // If here then we at least have the tuple size. Now check if the

            // index doesn't exceed that size.

            if tuple_member_index >= tuple_size as u64 {

                let select_expr_span = ctx.heap[node.expr_id].full_span();

                return Err(ParseError::new_error_at_span(

                    &ctx.module().source, select_expr_span, format!(

                        "element index {} is out of bounds, tuple has {} elements",

                        tuple_member_index, tuple_size

                    )

                ));

            }

            // Within bounds, set index on the type inference node

            let node = &mut self.infer_nodes[node_index];

            node.field_or_monomorph_index = tuple_member_index as i32;

        // If here then we know we can use `tuple_member_index`. We need to keep

        // computing the offset to the subtype, as its value changes during

        // inference

        let subject_type = &self.infer_nodes[subject_index].expr_type;

        let mut selected_member_start_index = 1; // start just after the InferenceTypeElement::Tuple

        for _ in 0..tuple_member_index {

            selected_member_start_index = InferenceType::find_subtree_end_idx(&subject_type.parts, selected_member_start_index);

        }

        let (progress_member, progress_subject) = self.apply_equal2_constraint(

            ctx, node_index, node_index, 0, subject_index, selected_member_start_idx

        )?;

        if progress_member { self.queue_node_parent(node_index); }

        if progress_subject { self.queue_node(subject_index); }

        return Ok(());

    }

    fn progress_inference_rule_literal_struct(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {

        let node = &self.infer_nodes[node_index];

        let rule = node.inference_rule.as_literal_struct();

        // For each of the fields in the literal struct, apply the type equality

        // constraint. If the literal is polymorphic, then we try to progress

        // their types during this process

        let mut poly_progress_section = self.poly_progress_buffer.start_section();

        for (field_index, field_node_index) in rule.element_indices.iter().copied().enumerate() {

            let field_expr_id = self.infer_nodes[field_node_index].expr_id;

            let (_, progress_field) = self.apply_polydata_equal2_constraint(

                ctx, node_index, field_expr_id, "struct field's",

                PolyDataTypeIndex::Associated(field_index), 0,

                field_node_index, 0, &mut poly_progress_section

            )?;

            if progress_field { self.queue_node(field_node_index); }

        }

        // Now we do the same thing for the struct literal expression (the type

        // of the struct itself).

        let (_, progress_literal_1) = self.apply_polydata_equal2_constraint(

            ctx, node_index, node.expr_id, "struct literal's",

            PolyDataTypeIndex::Returned, 0, node_index, 0, &mut poly_progress_section

        )?;

        // And the other way around: if any of our polymorphic variables are

        // more specific then they were before, then we forward that information

        // back to our struct/fields.

        for (field_index, field_node_index) in rule.element_indices.iter().copied().enumerate() {

            let progress_field = self.apply_polydata_polyvar_constraint(

                ctx, node_index, PolyDataTypeIndex::Associated(field_index),

                field_node_index, &poly_progress_section

            );

            if progress_field { self.queue_node(field_node_index); }

        }

        let progress_literal_2 = self.apply_polydata_polyvar_constraint(

            ctx, node_index, PolyDataTypeIndex::Returned,

            node_index, &poly_progress_section

        );

        if progress_literal_1 || progress_literal_2 { self.queue_node_parent(node_index); }

        poly_progress_section.forget();

    fn progress_inference_rule_literal_enum(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {

        let node = &self.infer_nodes[node_index];

        let rule = node.inference_rule.as_literal_enum();

        let mut poly_progress_section = self.poly_progress_buffer.start_section();

        // An enum literal type is simply, well, the enum's type. However, it

        // might still have polymorphic variables, hence the use of `PolyData`.

        let (_, progress_literal_1) = self.apply_polydata_equal2_constraint(

            ctx, node_index, node.expr_id, "enum literal's",

            PolyDataTypeIndex::Returned, 0, node_index, 0, &mut poly_progress_section

        )?;

        let progress_literal_2 = self.apply_polydata_polyvar_constraint(

            ctx, node_index, PolyDataTypeIndex::Returned, node_index, &poly_progress_section

        );

        if progress_literal_1 || progress_literal_2 { self.queue_node_parent(node_index); }

        poly_progress_section.forget();

        return Ok(());

    }

    fn progress_inference_rule_literal_union(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {

        let node = &self.infer_nodes[node_index];

        let rule = node.inference_rule.as_literal_union();

        // Infer type of any embedded values in the union variant. At the same

        // time progress the polymorphic variables associated with the union.

        let mut poly_progress_section = self.poly_progress_buffer.start_section();

        for (embedded_index, embedded_node_index) in rule.element_indices.iter().copied().enumerate() {

            let embedded_node_expr_id = self.infer_nodes[embedded_node_index].expr_id;

            let (_, progress_embedded) = self.apply_polydata_equal2_constraint(

                ctx, node_index, embedded_node_expr_id, "embedded value's",

                PolyDataTypeIndex::Associated(embedded_index), 0,

                embedded_node_index, 0, &mut poly_progress_section

            )?;

            if progress_embedded { self.queue_node(embedded_node_index); }

        }

        let (_, progress_literal_1) = self.apply_polydata_equal2_constraint(

            ctx, node_index, node.expr_id, "union's",

            PolyDataTypeIndex::Returned, 0, node_index, 0, &mut poly_progress_section

        )?;

        // Propagate progress in the polymorphic variables to the expressions

        // that constitute the union literal.

        for (embedded_index, embedded_node_index) in rule.element_indices.iter().copied().enumerate() {

            let progress_embedded = self.apply_polydata_polyvar_constraint(

                ctx, node_index, PolyDataTypeIndex::Associated(embedded_index),

                embedded_node_index, &poly_progress_section

            );

            if progress_embedded { self.queue_node(embedded_node_index); }

        }

        let progress_literal_2 = self.apply_polydata_polyvar_constraint(

            ctx, node_index, PolyDataTypeIndex::Returned, node_index, &poly_progress_section

        );

        if progress_literal_1 || progress_literal_2 { self.queue_node_parent(node_index); }

        poly_progress_section.forget();

        return Ok(());

    }

    fn progress_inference_rule_literal_array(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {

        let node = &self.infer_nodes[node_index];

        let rule = node.inference_rule.as_literal_array();

        // Apply equality rule to all of the elements that form the array

        let argument_node_indices = self.index_buffer.start_section_initialized(&rule.element_indices);

        let mut argument_progress_section = self.bool_buffer.start_section();

        self.apply_equal_n_constraint(ctx, node_index, &argument_node_indices, &mut argument_progress_section)?;

        debug_assert_eq!(argument_node_indices.len(), argument_progress_section.len());

        for argument_index in 0..argument_node_indices.len() {

            let argument_node_index = argument_node_indices[argument_index];

            let progress = argument_progress_section[argument_index];

            if progress { self.queue_node(argument_node_index); }

        }

        // If elements are of type `T`, then the array is of type `Array<T>`, so:

        let mut progress_literal = self.apply_template_constraint(ctx, node_index, &ARRAY_TEMPLATE)?;

        if argument_node_indices.len() != 0 {

            let argument_node_index = argument_node_indices[0];

            let (progress_literal_inner, progress_argument) = self.apply_equal2_constraint(

                ctx, node_index, node_index, 1, argument_node_index, 0

            )?;

            progress_literal = progress_literal || progress_literal_inner;

            // It is possible that the `Array<T>` has a more progress `T` then

            // the arguments. So in the case we progress our argument type we

            // simply queue this rule again

            if progress_argument { self.queue_expr(node_index); }

        }

        argument_node_indices.forget();

        argument_progress_section.forget();

        if progress_literal { self.queue_node_parent(node_index); }

        return Ok(());

    }

    fn progress_inference_rule_literal_tuple(&mut self, ctx: &Ctx, node_index: InferNodeIndex) -> Result<(), ParseError> {

        let node = &self.infer_nodes[node_index];

        let rule = node.inference_rule.as_literal_tuple();

    }

    /// have length N.

        &mut self, ctx: &Ctx, outer_node_index: InferNodeIndex,

        arguments: &ScopedSection<InferNodeIndex>, progress: &mut ScopedSection<bool>

        // Depending on the argument perform an early exit. This simplifies

        // later logic

        match arguments.len() {

        // We'll start doing pairwise inference for all of the inference nodes

        // (node[0] with node[1], then node[1] with node[2], then node[2] ...,

        // etc.), so when we're at the end we have `node[N-1]` as the most

        // progressed type.

        let mut last_index_requiring_inference = 0;

        for prev_argument_index in 0..arguments.len() - 1 {

            let next_argument_index = prev_argument_index + 1;

            let prev_node_index = arguments[prev_argument_index];

            let next_node_index = arguments[next_argument_index];

            let (prev_progress, next_progress) = self.apply_equal2_constraint(

                ctx, outer_node_index, prev_node_index, 0, next_node_index, 0

            )?;

            if prev_progress {

                // Previous node is progress, so every type in front of it needs

                // to be reinferred.

                progress[prev_argument_index] = true;

                last_index_requiring_inference = prev_argument_index;

            progress[next_argument_index] = next_progress;

        // Apply inference using the most progressed type (the last one) to the

        // ones that did not obtain this information during the inference

        // process.

        let last_argument_node_index = arguments[arguments.len() - 1];

        let last_argument_type: *mut _ = &mut self.infer_nodes[last_argument_node_index].expr_type;

        for argument_index in 0..last_index_requiring_inference {

            // We can cheat, we know the LHS is less specific than the right

            // hand side, so:

            let argument_node_index = arguments[argument_index];

            let argument_type = &mut self.infer_nodes[argument_node_index].expr_type;

                // safety: we're dealing with different vectors, so cannot alias

                argument_type.replace_subtree(0, &(*last_argument_type).parts);

            progress[argument_index] = true;