Do not subject protocol Self to accessibility diagnostics

It is currently impossible to declare a protocol as unavailable and
still ship valid Swift code because while typechecking protocol Self,
we attempt to do an accessibility check on its generic signature and
find that, surprise surprise, the protocol is unavailable.  Rather than
plumb another parameter through Sema, this patch piggybacks on existing
behavior of AllowPotentiallyUnavailableProtocol to disable the check
for protocol self.

Author: CodaFi

lib/Sema/TypeCheckDecl.cpp

@@ -271,13 +271,18 @@ void TypeChecker::checkInheritanceClause(Decl *decl,
   } else if (auto ext = dyn_cast<ExtensionDecl>(decl)) {
     DC = ext;
     options |= TR_GenericSignature | TR_InheritanceClause;
-  } else if (isa<GenericTypeParamDecl>(decl)) {
+  } else if (auto GP = dyn_cast<GenericTypeParamDecl>(decl)) {
     // For generic parameters, we want name lookup to look at just the
     // signature of the enclosing entity.
     DC = decl->getDeclContext();
     if (auto nominal = dyn_cast<NominalTypeDecl>(DC)) {
       DC = nominal;
       options |= TR_GenericSignature;
+      // When looking up protocol 'Self' accessibility checks are disabled to
+      // head off spurious unavailable diagnostics.
+      if (isa<ProtocolDecl>(DC) && GP->isProtocolSelf()) {
+        options |= TR_AllowUnavailable;
+      }
     } else if (auto ext = dyn_cast<ExtensionDecl>(DC)) {
       DC = ext;
       options |= TR_GenericSignature;

test/attr/attr_availability.swift

@@ -31,6 +31,13 @@ typealias int = Int // expected-note {{'int' has been explicitly marked unavaila
 @available(*, unavailable, renamed: "Float")
 typealias float = Float // expected-note {{'float' has been explicitly marked unavailable here}}
 
+protocol MyNewerProtocol {}
+
+@available(*, unavailable, renamed: "MyNewerProtocol")
+protocol MyOlderProtocol {} // expected-note {{'MyOlderProtocol' has been explicitly marked unavailable here}}
+
+extension Int: MyOlderProtocol {} // expected-error {{'MyOlderProtocol' has been renamed to 'MyNewerProtocol'}} 
+
 struct MyCollection<Element> {
   @available(*, unavailable, renamed: "Element")
   typealias T = Element // expected-note 2{{'T' has been explicitly marked unavailable here}}