We have been discussing using anyref as an exception type (or its supertype) recently, as @eholk mentioned in #9 (comment).

The current exception handling proposal imposes several difficulties (WebAssembly/exception-handling#30 and WebAssembly/exception-handling#31), and allowing opaque exception objects to be stored in locals and be dynamically type-tested can solve most of those problems. (They don't have to be stored in linear memory.) And the proposed anyref as WASM value (#9) type sounds like it satisfies many requirements.

One thing is, we use a 'tagged value' to represent an exception object. An exception object is a pair of a tag and a list of values. Definitions of the related terminologies are here. Tags can be used many ways, possibly to denote types (int, MyException&, ...) or languages (C++, JavaScript, ...). In C++ exception support, we are using them to denote languages (we can't do types with C++ because of inheritance and such): so for example a specific tag can mean C++, the other tag can mean JavaScript, etc.

Can we treat anyref as tagged values in general? It might be possible for most of non-exception objects to have the same predefined tag, making them essentially tagless. This way, we would also need a match instruction that dynamically tells if the current object (on stack) has the specified tag or not.

Or, can we make the tagged value type as a subtype of anyref? In that way we would also need some instructions like isinstanceof as suggested in #4, as well as also the match to dynamically test the tag. This way we need to introduce supertype/subtype hierarchy in the system.

cc @eholk @dschuff @KarlSchimpf