Merge pull request swiftlang#34499 from slavapestov/opaque-result-with-existential-base

Sema: Ban references to protocol extension members with opaque result types on an existential base

Author: slavapestov

lib/AST/Decl.cpp

@@ -4795,6 +4795,11 @@ findProtocolSelfReferences(const ProtocolDecl *proto, Type type,
     return info;
   }
 
+  // Opaque result types of protocol extension members contain an invariant
+  // reference to 'Self'.
+  if (type->is<OpaqueTypeArchetypeType>())
+    return SelfReferenceInfo::forSelfRef(SelfReferencePosition::Invariant);
+
   // A direct reference to 'Self'.
   if (proto->getSelfInterfaceType()->isEqual(type))
     return SelfReferenceInfo::forSelfRef(position);

test/type/opaque.swift

@@ -485,3 +485,28 @@ protocol SomeProtocolA {}
 protocol SomeProtocolB {}
 struct SomeStructC: SomeProtocolA, SomeProtocolB {}
 let someProperty: SomeProtocolA & some SomeProtocolB = SomeStructC() // expected-error {{'some' should appear at the beginning of a composition}}{{35-40=}}{{19-19=some }}
+
+// An opaque result type on a protocol extension member effectively
+// contains an invariant reference to 'Self', and therefore cannot
+// be referenced on an existential type.
+
+protocol OpaqueProtocol {}
+extension OpaqueProtocol {
+  var asSome: some OpaqueProtocol { return self }
+  func getAsSome() -> some OpaqueProtocol { return self }
+  subscript(_: Int) -> some OpaqueProtocol { return self }
+}
+
+func takesOpaqueProtocol(existential: OpaqueProtocol) {
+  // this is not allowed:
+  _ = existential.asSome // expected-error{{member 'asSome' cannot be used on value of protocol type 'OpaqueProtocol'; use a generic constraint instead}}
+  _ = existential.getAsSome() // expected-error{{member 'getAsSome' cannot be used on value of protocol type 'OpaqueProtocol'; use a generic constraint instead}}
+  _ = existential[0] // expected-error{{member 'subscript' cannot be used on value of protocol type 'OpaqueProtocol'; use a generic constraint instead}}
+}
+
+func takesOpaqueProtocol<T : OpaqueProtocol>(generic: T) {
+  // these are all OK:
+  _ = generic.asSome
+  _ = generic.getAsSome()
+  _ = generic[0]
+}