Sema: Diagnose unavailability of associated type decls.

It should not be possible to mark an associated type declaration unavailable
since doing so does not have an effect on how a conformance interacts with its
associated types.

Author: tshortli

lib/Sema/TypeCheckAttr.cpp

@@ -4982,6 +4982,11 @@ TypeChecker::diagnosticIfDeclCannotBeUnavailable(const Decl *D) {
     return Diagnostic(diag::availability_decl_no_unavailable, D);
   }
 
+  // The conformance checker does not know what to do with unavailable
+  // associated types.
+  if (auto *AT = dyn_cast<AssociatedTypeDecl>(D))
+    return Diagnostic(diag::availability_decl_no_unavailable, D);
+
   if (auto *VD = dyn_cast<VarDecl>(D)) {
     if (!VD->hasStorageOrWrapsStorage())
       return std::nullopt;

test/decl/protocol/associated_type_availability.swift

@@ -84,3 +84,11 @@ extension ModelP2 {
   // expected-error@-1{{'B' is only available in macOS 14 or newer}}
   // expected-note@-2{{add @available attribute to enclosing instance method}}
 }
+
+protocol P3 {
+  @available(macOS, obsoleted: 12) // expected-error{{associated type cannot be marked unavailable with '@available'}}
+  associatedtype A1
+
+  @available(macOS, obsoleted: 99) // FIXME: this should probably be diagnosed
+  associatedtype A2
+}

test/decl/protocol/req/unavailable.swift

@@ -91,3 +91,11 @@ struct SelfT: Unavail { // expected-error {{type 'SelfT' does not conform to pro
   // expected-error@-1 {{unavailable instance method 'req()' was used to satisfy a requirement of protocol 'Unavail': write it yourself}}
   typealias T = SelfT
 }
+
+protocol UnavailableAssoc {
+  @available(*, unavailable) // expected-error {{associated type cannot be marked unavailable with '@available'}}
+  associatedtype A1
+
+  @available(swift, introduced: 99) // expected-error {{associated type cannot be marked unavailable with '@available'}}
+  associatedtype A2
+}