C++: `undeclared identifier` errors building generated code

[itowlson]

The code for this issue is at: https://github.com/itowlson/spin-cpp-wasip2-test

I am trying to build an HTTP handler using WASI 0.2. I used the C++ binding generator from wit-bindgen 0.57.1:

wit-bindgen cpp ./wit/ --out-dir bindings -w http-trigger

and the WASI-SDK 33 compiler:

$(WASI_SDK_PATH)/bin/clang++ -std=c++20 -I bindings -I expected -target wasm32-wasip2 test.cpp bindings/http_trigger.cpp bindings/http_trigger_component_type.o -mexec-model=reactor -o test.wasm

I have patched up expected and wit.h, which has gotten rid of most the first tranche of errors. However, I still get a lot of build errors for generated types for some of my WITs (I think for the Spin APIs, rather than part of WASI), e.g.

./bindings/http_trigger_cpp.h:300:42: error: use of undeclared identifier 'Error'
  300 |         static std::expected<Connection, Error> Open(std::string_view database);

(The WIT interface here, an API for SQLite, defines an error variant. The .h file does contain an Error type, albeit after this line of code.)

Am I doing something wrong (e.g. wit-bindgen or clang options)? How can I get this generated code to build? Thanks!

(And I'm happy to offer a README for the C++ generator if I can get this working. I could only find one for C so far.)