Sema: Allow unavailable protocol witnesses in unavailable nominals or extensions

lib/Sema/TypeCheckProtocol.cpp

@@ -1694,7 +1694,23 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
 
   if (match.Witness->getAttrs().isUnavailable(getASTContext()) &&
       !requirement->getAttrs().isUnavailable(getASTContext())) {
-    return CheckKind::WitnessUnavailable;
+    auto nominalOrExtensionIsUnavailable = [&]() {
+      if (auto extension = dyn_cast<ExtensionDecl>(DC)) {
+        if (extension->getAttrs().isUnavailable(getASTContext()))
+          return true;
+      }
+
+      if (auto adoptingNominal = DC->getSelfNominalTypeDecl()) {
+        if (adoptingNominal->getAttrs().isUnavailable(getASTContext()))
+          return true;
+      }
+
+      return false;
+    };
+
+    // Allow unavailable nominals or extension to have unavailable witnesses.
+    if (!nominalOrExtensionIsUnavailable())
+      return CheckKind::WitnessUnavailable;
   }
 
   return CheckKind::Success;

test/decl/protocol/req/unavailable.swift

@@ -41,6 +41,28 @@ struct ConformsToP2 {
 extension ConformsToP2: P {} // expected-error{{type 'ConformsToP2' does not conform to protocol 'P'}}
 // expected-error@-1 {{unavailable instance method 'foo(bar:)' was used to satisfy a requirement of protocol 'P'}}
 
+@available(*, unavailable)
+struct ConformsToP3: P {
+  func foo(bar: Foo) { }
+}
+
+// Ok, an unavailable decl should be allowed to witness a requirement when the
+// conforming type is itself unavailable.
+@available(*, unavailable)
+struct ConformsToP4: P {
+  @available(*, unavailable)
+  func foo(bar: Foo) { }
+}
+
+struct ConformsToP5 {}
+
+// Ok, an unavailable decl should be allowed to witness a requirement when the
+// conformance extension is itself unavailable.
+@available(*, unavailable)
+extension ConformsToP5: P {
+  @available(*, unavailable)
+  func foo(bar: Foo) { }
+}
 
 // Include message string from @available attribute if provided
 protocol Unavail {