Opaque result types

Joe_Groff · January 17, 2019, 12:08am

I have a minimal prototype implementation of opaque result types in this pull request:

Limitations include:

The opaque type is spelled __opaque Protocol or __opaque ProtocolA & ProtocolB [& ProtocolC...] as placeholder spelling.
Opaque types can only appear as the return type of func declarations, not properties or closures.
where clause constraints on the opaque type are not yet supported, nor are conditional conformances on the opaque type. However, the protocols in the __opaque type can be arbitrary protocols, with any number of associated type or Self type requirements. __opaque types can in turn be inferred as associated types of protocol conformances.
The runtime and resilience aspects are not yet implemented. Changing the underlying type of a public opaque type will break ABI.

Some known issues:

Because of the missing runtime support, I substitute out opaque types during SILGen in a rather hacky unprincipled way. It's likely that the optimizer, particularly generic specialization, may still cause crashes.
The type checker currently crashes if there's a type error in the underlying type, such as if it does not conform to the required protocols.
Diagnostics involving opaque types will print the type with a compiler-internal representation involving the original decl name and generic arguments, instead of anything human-understandable.