[Property wrappers] Diagnose unavailable wrappedValue

Fixes rdar://problem/53678041 and rdar://problem/57676337; the former was
because we diagnosed unavailable wrappedValue at the null source location
originall, then started suppressing that diagnostic altogether. Produce a
proper diagnostic now.

Author: DougGregor

lib/Sema/TypeCheckStorage.cpp

@@ -17,6 +17,7 @@
 
 #include "CodeSynthesis.h"
 #include "TypeChecker.h"
+#include "TypeCheckAvailability.h"
 #include "TypeCheckDecl.h"
 #include "TypeCheckType.h"
 #include "swift/AST/ASTContext.h"
@@ -694,7 +695,20 @@ static Expr *buildStorageReference(AccessorDecl *accessor,
     // Perform accesses to the wrappedValues along the composition chain.
     for (unsigned i : range(firstWrapperIdx, lastWrapperIdx)) {
       auto wrapperInfo = var->getAttachedPropertyWrapperTypeInfo(i);
-      underlyingVars.push_back(wrapperInfo.valueVar);
+      auto wrappedValue = wrapperInfo.valueVar;
+
+      // Check for availability of wrappedValue.
+      if (accessor->getAccessorKind() == AccessorKind::Get ||
+          accessor->getAccessorKind() == AccessorKind::Read) {
+        if (auto *attr = wrappedValue->getAttrs().getUnavailable(ctx)) {
+          diagnoseExplicitUnavailability(
+              wrappedValue,
+              var->getAttachedPropertyWrappers()[i]->getRangeWithAt(),
+              var->getDeclContext(), nullptr);
+        }
+      }
+
+      underlyingVars.push_back(wrappedValue);
     }
     semantics = AccessSemantics::DirectToStorage;
     selfAccessKind = SelfAccessorKind::Peer;

test/decl/var/property_wrappers.swift

@@ -9,12 +9,11 @@ struct Wrapper<T> {
   init(stored: T) {
     self._stored = stored
   }
-  
+
   var wrappedValue: T {
     get { _stored }
     set { _stored = newValue }
   }
-  
 }
 
 @propertyWrapper
@@ -837,6 +836,42 @@ struct UsesExplicitClosures {
   var y: Int
 }
 
+// ---------------------------------------------------------------------------
+// Enclosing instance diagnostics
+// ---------------------------------------------------------------------------
+@propertyWrapper
+struct Observable<Value> {
+  private var stored: Value
+
+  init(wrappedValue: Value) {
+    self.stored = wrappedValue
+  }
+
+  @available(*, unavailable, message: "must be in a class")
+  var wrappedValue: Value { // expected-note{{'wrappedValue' has been explicitly marked unavailable here}}
+    get { fatalError("called wrappedValue getter") }
+    set { fatalError("called wrappedValue setter") }
+  }
+
+  static subscript<EnclosingSelf>(
+      _enclosingInstance observed: EnclosingSelf,
+      wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>,
+      storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Self>
+    ) -> Value {
+    get {
+      observed[keyPath: storageKeyPath].stored
+    }
+    set {
+      observed[keyPath: storageKeyPath].stored = newValue
+    }
+  }
+}
+
+struct MyObservedValueType {
+  @Observable // expected-error{{'wrappedValue' is unavailable: must be in a class}}
+  var observedProperty = 17
+}
+
 // ---------------------------------------------------------------------------
 // Miscellaneous bugs
 // ---------------------------------------------------------------------------