exclude private memberwise init

Author: hamishknight

include/swift/Basic/Features.def

@@ -544,6 +544,10 @@ EXPERIMENTAL_FEATURE(DefaultIsolationPerFile, false)
 /// Enable @_lifetime attribute
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(Lifetimes, true)
 
+/// Excludes 'private' properties with initial values from the memberwise
+/// initializer.
+EXPERIMENTAL_FEATURE(ExcludePrivateFromMemberwiseInit, false)
+
 /// Allow macro based aliases to be imported into Swift
 EXPERIMENTAL_FEATURE(ImportMacroAliases, true)
 

lib/AST/Decl.cpp

@@ -8412,10 +8412,17 @@ bool VarDecl::isMemberwiseInitialized(bool preferDeclaredProperties) const {
   // If this is a stored property, and not a backing property in a case where
   // we only want to see the declared properties, it can be memberwise
   // initialized.
-  if (hasStorage() && preferDeclaredProperties &&
-      isBackingStorageForDeclaredProperty(this))
-    return false;
-
+  if (hasStorage()) {
+    if (isBackingStorageForDeclaredProperty(this)) {
+      if (preferDeclaredProperties)
+        return false;
+    } else if (getASTContext().LangOpts.hasFeature(Feature::ExcludePrivateFromMemberwiseInit)) {
+      // Private variables with initial values are never memberwise initialized.
+      if (getFormalAccess() == AccessLevel::Private)
+        if (hasInitialValue() || getParentPatternBinding()->isDefaultInitializable())
+          return false;
+    }
+  }
   // If this is a computed property with `init` accessor, it's
   // memberwise initializable when it could be used to initialize
   // other stored properties.

lib/AST/FeatureSet.cpp

@@ -127,6 +127,7 @@ UNINTERESTING_FEATURE(NonisolatedNonsendingByDefault)
 UNINTERESTING_FEATURE(KeyPathWithMethodMembers)
 UNINTERESTING_FEATURE(ImportMacroAliases)
 UNINTERESTING_FEATURE(NoExplicitNonIsolated)
+UNINTERESTING_FEATURE(ExcludePrivateFromMemberwiseInit)
 
 // TODO: Return true for inlinable function bodies with module selectors in them
 UNINTERESTING_FEATURE(ModuleSelector)