Synthesized reflective proxies have wrong This type for hijacked classes

[sjrd]

When the base linker's MethodSynthesizer generates reflective proxies in hijacked classes, it creates This nodes with type ClassType(HijackedClass). This is not valid in the IR; they should have the corresponding primitive type. For IR coming from ClassDefs, this is checked by the ClassDefChecker. Synthesized methods, however, are not checked by the ClassDefChecker and therefore bypass the problem.

I was not able to produce a user-visible bug because of this. This seems to be because

• The only hijacked class for which the tpe of This node matters is Character (in terms of how the Emitter works)
• Character has a direct implementation of every method that is not declared on Any

Therefore, we never synthesize a reflective proxy in Character, and we never run into the issue in practice.

However, we discovered this issue while working on the WebAssembly backend, for which all hijacked classes need to have This types with the correct type in order to produce well-typed Wasm code. See the workaround here:
https://github.com/tanishiking/scala-wasm/blob/c9b8270e3da1a8f5335b18fc9bc79969e7541ec5/wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/WasmExpressionBuilder.scala#L1684-L1692

The same problem likely exists in default bridges in theory. In practice, though, none of the hijacked classes inherit from any default method that they don't override. So it never even leads to any invalid default bridge being generated, let alone one that we can exploit to produce a bug.