[Sema] Formalize a way to check whether storage is initializable

Previously only stored properties could be initializable but with
introduction of init accessors computed properties gained an ability
to specify initialzer expression and participation in memberwise
initialization for structs.

Author: xedin

include/swift/AST/Decl.h

@@ -5321,6 +5321,12 @@ class AbstractStorageDecl : public ValueDecl {
   /// with it.
   bool hasInitAccessor() const;
 
+  /// Return true if this is a property that either has storage
+  /// or init accessor associated with it.
+  bool supportsInitialization() const {
+    return hasStorage() || hasInitAccessor();
+  }
+
   /// Return true if this storage has the basic accessors/capability
   /// to be mutated.  This is generally constant after the accessors are
   /// installed by the parser/importer/whatever.

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2289,7 +2289,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         if (PBD->isInitialized(i)) {
           // Add the attribute that preserves the "has an initializer" value
           // across module generation, as required for TBDGen.
-          if (var->hasStorage() &&
+          if (var->supportsInitialization() &&
               !var->getAttrs().hasAttribute<HasInitialValueAttr>()) {
             var->getAttrs().add(new (Ctx)
                                     HasInitialValueAttr(/*IsImplicit=*/true));

lib/Sema/TypeCheckMacros.cpp

@@ -1319,9 +1319,11 @@ Optional<unsigned> swift::expandAccessors(
     !accessorMacroOnlyIntroducesObservers(macro, roleAttr);
   if (foundNonObservingAccessor) {
     // If any non-observing accessor was added, mark the initializer as
-    // subsumed.
+    // subsumed unless it has init accessor, because the initializer in
+    // such cases could be used for memberwise initialization.
     if (auto var = dyn_cast<VarDecl>(storage)) {
-      if (auto binding = var->getParentPatternBinding()) {
+      if (auto binding = var->getParentPatternBinding();
+          !var->getAccessor(AccessorKind::Init)) {
         unsigned index = binding->getPatternEntryIndexForVarDecl(var);
         binding->setInitializerSubsumed(index);
       }