How does the compiler look for opaque types?

Development Compiler
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1

I was browsing the Swift source code (in C++ using the LLVM interface libraries). I was reading the file about recognizing type expressions and another file about recognizing value expressions. I was thinking about trying to add a type construction that involves an opaque type. I was ready to think about parsing a "some" followed by another type expression, but I got worried about what if the opaque type is hidden behind a type alias. I recently tried a (macOS) playground, and it doesn't allow making an opaque type into an alias.

That's good from a user viewpoint. But now I'm wondering how opaque types are identified. Does each type expression that allows an opaque type within it search for a "some" followed by another type expression? Or is there a common code all those experiences share? And if it's shared code (I hope so), where is it?

2

In some sense yes. (I don't think they are allowed in nested positions though.). You can take a look at this method for more details.

OpaqueTypeDecl *
OpaqueResultTypeRequest::evaluate(Evaluator &evaluator,
                                  ValueDecl *originatingDecl) const

It has one of the calls to OpaqueTypeArchetypeType::get which is a factory method.

You can also take a look at getOpaqueResultTypeDecl() and it's callers. (Xcode trips over at displaying the call hierarchy for OpaqueResultTypeRequest::evaluate, so you might think nothing is calling it.)