fix template union forward declaration, close #1768

[Danielmelody]

No description provided.

[Danielmelody]

Any review progress?

[tgross35]

This one LGTM @emilio

[emilio]

Why is this not an issue for structs?

[aleb]

Below is a sample union XXX item failing. The panic happens because no layout can be obtained from the item. This happens because layout: None and (in case of Unions) fields: []. In case of struct XXX, the codepath is different, the layout is simply not mandatory. See audocxx-bindgen/.../codegen/mod.rs:

impl CodeGenerator for CompInfo {

    fn codegen(...) {
        ...
        if is_opaque {
            ...
        } else if !is_union && !zero_sized {
            ... struct handled here, basically nothing is done without a `layout`
        } else if is_union && !forward_decl {
            // TODO(emilio): It'd be nice to unify this with the struct path
            // above somehow.
            let layout = layout.expect("Unable to get layout information?");
            ...
        }

        ...
    }

  cargo:warning=MESSAGE:<CompInfo as CodeGenerator>::codegen: item = Item { id: ItemId(23), local_id: OnceCell(<uninit>), next_child_local_id: Cell { value: 1 }, canonical_name: OnceCell(<uninit>), path_for_allowlisting: OnceCell(["root", "Sandwich"]), comment: None, annotations: Annotations { opaque: false, hide: false, use_instead_of: None, disallow_copy: false, disallow_debug: false, disallow_default: false, must_use_type: false, visibility_kind: None, accessor_kind: None, constify_enum_variant: false, derives: [] }, parent_id: ItemId(0), kind: Type(Type { name: Some("Sandwich"), layout: None, kind: Comp(CompInfo { kind: Union, visibility: Public, fields: After { fields: [], has_bitfield_units: false }, template_params: [TypeId(ItemId(24))], methods: [], constructors: [], destructor: None, base_members: [], inner_types: [], inner_vars: [], has_own_virtual_method: false, has_destructor: false, has_nonempty_base: false, has_non_type_template_params: false, has_unevaluable_bit_field_width: false, packed_attr: false, found_unknown_attr: false, is_forward_declaration: false }), is_const: false }), location: Some(include/aaa.hh:3:7) }
  cargo:warning=MESSAGE:Offset: _address: 0 -> 1
  thread 'main' panicked at /home/minion/.cargo/registry/src/index.crates.io-6f17d22bba15001f/autocxx-bindgen-0.69.5/codegen/mod.rs:2291:33:
  Unable to get layout information?

(obtained from https://github.com/aleb/autocxx-test/tree/v1 with RUST_LOG=debug RUST_BACKTRACE=full cargo build -r with this include.h)