[Strict memory safety] Type alias types are only unsafe if their underlying type is

(cherry picked from commit 53ca902)

Author: DougGregor

include/swift/AST/Types.h

@@ -318,6 +318,11 @@ class RecursiveTypeProperties {
     Bits &= ~HasDependentMember;
   }
 
+  /// Remove the IsUnsafe property from this set.
+  void removeIsUnsafe() {
+    Bits &= ~IsUnsafe;
+  }
+  
   /// Test for a particular property in this set.
   bool operator&(Property prop) const {
     return Bits & prop;

lib/AST/ASTContext.cpp

@@ -3543,6 +3543,12 @@ TypeAliasType *TypeAliasType::get(TypeAliasDecl *typealias, Type parent,
   auto &ctx = underlying->getASTContext();
   auto arena = getArena(properties);
 
+  // Typealiases can't meaningfully be unsafe; it's the underlying type that
+  // matters.
+  properties.removeIsUnsafe();
+  if (underlying->isUnsafe())
+    properties |= RecursiveTypeProperties::IsUnsafe;
+
   // Profile the type.
   llvm::FoldingSetNodeID id;
   TypeAliasType::Profile(id, typealias, parent, genericArgs, underlying);
@@ -4212,8 +4218,7 @@ static RecursiveTypeProperties getRecursivePropertiesAsParent(Type type) {
 
   // Drop the "unsafe" bit. We have to recompute it without considering the
   // enclosing nominal type.
-  properties = RecursiveTypeProperties(
-      properties.getBits() & ~static_cast<unsigned>(RecursiveTypeProperties::IsUnsafe));
+  properties.removeIsUnsafe();
 
   // Check generic arguments of parent types.
   while (type) {

lib/Sema/TypeCheckUnsafe.cpp

@@ -380,7 +380,7 @@ void swift::diagnoseUnsafeType(ASTContext &ctx, SourceLoc loc, Type type,
   if (!ctx.LangOpts.hasFeature(Feature::StrictMemorySafety))
     return;
 
-  if (!type->getCanonicalType()->isUnsafe())
+  if (!type->isUnsafe())
     return;
 
   // Look for a specific @unsafe nominal type along the way.