[5.1] [SE-0258] Trigger synthesis of _foo/$foo from name lookup.

lib/Sema/CodeSynthesis.cpp

@@ -1523,6 +1523,9 @@ static VarDecl *synthesizePropertyWrapperStorageWrapperProperty(
   Identifier name = ctx.getIdentifier(nameBuf);
 
   // Determine the type of the property.
+  if (!wrapperVar->hasInterfaceType()) {
+    static_cast<TypeChecker &>(*ctx.getLazyResolver()).validateDecl(wrapperVar);
+  }
   Type propertyType = wrapperType->getTypeOfMember(
       var->getModuleContext(), wrapperVar,
       wrapperVar->getValueInterfaceType());

lib/Sema/TypeCheckDecl.cpp

@@ -5506,6 +5506,25 @@ void TypeChecker::synthesizeMemberForLookup(NominalTypeDecl *target,
       if (!evaluateTargetConformanceTo(decodableProto))
         (void)evaluateTargetConformanceTo(encodableProto);
     }
+
+    if ((baseName.getIdentifier().str().startswith("$") ||
+         baseName.getIdentifier().str().startswith("_")) &&
+        baseName.getIdentifier().str().size() > 1) {
+      // $- and _-prefixed variables can be generated by properties that have
+      // attached property wrappers.
+      auto originalPropertyName =
+        Context.getIdentifier(baseName.getIdentifier().str().substr(1));
+      for (auto member : target->lookupDirect(originalPropertyName)) {
+        if (auto var = dyn_cast<VarDecl>(member)) {
+          if (var->hasAttachedPropertyWrapper()) {
+            auto sourceFile = var->getDeclContext()->getParentSourceFile();
+            if (sourceFile && sourceFile->Kind != SourceFileKind::Interface)
+              (void)var->getPropertyWrapperBackingPropertyInfo();
+          }
+        }
+      }
+    }
+
   } else {
     auto argumentNames = member.getArgumentNames();
     if (member.isCompoundName() && argumentNames.size() != 1)

test/multifile/Inputs/rdar51725203.swift

@@ -0,0 +1,23 @@
+@propertyWrapper
+struct Wrapper<Value> {
+  var wrappedValue: Value
+
+  var projectedValue: Wrapper<Value> {
+    get { self }
+    set { self = newValue }
+  }
+
+  init(initialValue: Value) {
+    wrappedValue = initialValue
+  }
+}
+
+struct StructModel {
+  @Wrapper var foo: Int
+  @Wrapper var bar: Int // expected-note{{'_bar' declared here}}
+}
+
+class ClassModel {
+  @Wrapper var foo = 17
+  @Wrapper var bar = 17 // expected-note{{'_bar' declared here}}
+}

test/multifile/property-wrappers-rdar51725203.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-frontend -typecheck -verify -primary-file %s %S/Inputs/rdar51725203.swift
+
+func test(sm: StructModel, cm: ClassModel) {
+  _ = sm.$foo
+  _ = cm.$foo
+
+  _ = sm._bar // expected-error{{'_bar' is inaccessible due to 'private' protection level}}
+  _ = cm._bar // expected-error{{'_bar' is inaccessible due to 'private' protection level}}
+}