[Property Wrappers] Improve error recovery in buildStorageReference.

Bail out if the backing storage has an error type. This can happen
if there's an invalid redeclaration of the property wrapper, for
example.

Author: hborla

lib/Sema/TypeCheckStorage.cpp

@@ -676,7 +676,7 @@ static Expr *buildStorageReference(AccessorDecl *accessor,
     auto *backing = var->getPropertyWrapperBackingProperty();
 
     // Error recovery.
-    if (!backing)
+    if (!backing || backing->isInvalid())
       return nullptr;
 
     storage = backing;
@@ -719,7 +719,7 @@ static Expr *buildStorageReference(AccessorDecl *accessor,
     auto *backing = var->getPropertyWrapperBackingProperty();
 
     // Error recovery.
-    if (!backing)
+    if (!backing || backing->isInvalid())
       return nullptr;
 
     storage = backing;

test/decl/var/property_wrappers.swift

@@ -825,6 +825,26 @@ struct S {
   var wrappedValue: Int
 }
 
+// ---------------------------------------------------------------------------
+// Invalid redeclaration
+// ---------------------------------------------------------------------------
+@propertyWrapper
+struct WrapperWithProjectedValue<T> {
+  var wrappedValue: T
+  var projectedValue: T { return wrappedValue }
+}
+
+class TestInvalidRedeclaration {
+  @WrapperWithProjectedValue var i = 17
+  // expected-note@-1 {{'i' previously declared here}}
+  // expected-note@-2 {{'$i' previously declared here}}
+  // expected-note@-3 {{'_i' previously declared here}}
+  @WrapperWithProjectedValue var i = 39
+  // expected-error@-1 {{invalid redeclaration of 'i'}}
+  // expected-error@-2 {{invalid redeclaration of '$i'}}
+  // expected-error@-3 {{invalid redeclaration of '_i'}}
+}
+
 // ---------------------------------------------------------------------------
 // Closures in initializers
 // ---------------------------------------------------------------------------