AST-verify that 'open' is only used on classes and overridable members (swiftlang#15996)

...and fix places where it was being used inappropriately.

- Don't use 'open' on non-class members in the importer.
- Use the existing 'copyFormalAccessFrom' instead of an ad hoc version
  for synthesized typealiases for protocol conformances. (This can
  change 'internal' down to 'fileprivate', but only where the
  enclosing type was already 'private' or 'fileprivate'.)
- Fix 'copyFormalAccessFrom' to not copy '@usableFromInline' onto
  declarations that don't support it (namely, the above typealiases).

This should have no visible effect in practice.

Author: jrose-apple

include/swift/AST/Decl.h

@@ -6703,7 +6703,8 @@ inline bool ValueDecl::isImportAsMember() const {
 inline bool Decl::isPotentiallyOverridable() const {
   if (isa<VarDecl>(this) ||
       isa<SubscriptDecl>(this) ||
-      isa<FuncDecl>(this)) {
+      isa<FuncDecl>(this) ||
+      isa<DestructorDecl>(this)) {
     return getDeclContext()->getAsClassOrClassExtensionContext();
   } else {
     return false;

lib/AST/ASTVerifier.cpp

@@ -2114,11 +2114,20 @@ class Verifier : public ASTWalker {
     }
 
     void verifyChecked(ValueDecl *VD) {
-      if (!VD->hasAccess() && !VD->getDeclContext()->isLocalContext() &&
-          !isa<GenericTypeParamDecl>(VD) && !isa<ParamDecl>(VD)) {
-        dumpRef(VD);
-        Out << " does not have access";
-        abort();
+      if (VD->hasAccess()) {
+        if (VD->getFormalAccess() == AccessLevel::Open &&
+            !isa<ClassDecl>(VD) && !VD->isPotentiallyOverridable()) {
+          Out << "decl cannot be 'open'\n";
+          VD->dump(Out);
+          abort();
+        }
+      } else {
+        if (!VD->getDeclContext()->isLocalContext() &&
+            !isa<GenericTypeParamDecl>(VD) && !isa<ParamDecl>(VD)) {
+          dumpRef(VD);
+          Out << " does not have access";
+          abort();
+        }
       }
 
       // Make sure that there are no archetypes in the interface type.

lib/AST/Decl.cpp

@@ -2282,7 +2282,8 @@ void ValueDecl::copyFormalAccessFrom(const ValueDecl *source,
   // Inherit the @usableFromInline attribute.
   if (source->getAttrs().hasAttribute<UsableFromInlineAttr>() &&
       !getAttrs().hasAttribute<UsableFromInlineAttr>() &&
-      !getAttrs().hasAttribute<InlinableAttr>()) {
+      !getAttrs().hasAttribute<InlinableAttr>() &&
+      DeclAttribute::canAttributeAppearOnDecl(DAK_UsableFromInline, this)) {
     auto &ctx = getASTContext();
     auto *clonedAttr = new (ctx) UsableFromInlineAttr(/*implicit=*/true);
     getAttrs().add(clonedAttr);

lib/ClangImporter/ImportDecl.cpp

@@ -105,9 +105,9 @@ static bool isInSystemModule(DeclContext *D) {
   return cast<ClangModuleUnit>(D->getModuleScopeContext())->isSystemModule();
 }
 
-static AccessLevel getOverridableAccessLevel(DeclContext *dc) {
-  return (dc->getAsProtocolOrProtocolExtensionContext()
-            ? AccessLevel::Public : AccessLevel::Open);
+static AccessLevel getOverridableAccessLevel(const DeclContext *dc) {
+  return (dc->getAsClassOrClassExtensionContext()
+            ? AccessLevel::Open : AccessLevel::Public);
 }
 
 /// Create a typedpattern(namedpattern(decl))

lib/Sema/TypeCheckProtocol.cpp

@@ -2178,9 +2178,7 @@ void ConformanceChecker::recordTypeWitness(AssociatedTypeDecl *assocType,
       // a typealias added for an internal protocol shouldn't need to be
       // public---but that can be problematic if the same type conforms to two
       // protocols with different access levels.
-      AccessLevel aliasAccess = nominal->getFormalAccess();
-      aliasAccess = std::max(aliasAccess, AccessLevel::Internal);
-      aliasDecl->setAccess(aliasAccess);
+      aliasDecl->copyFormalAccessFrom(nominal, /*sourceIsParentContext*/true);
 
       if (nominal == DC) {
         nominal->addMember(aliasDecl);

test/attr/accessibility_print.swift

@@ -345,8 +345,8 @@ public class IC_PublicAssocTypeImpl: IA_PublicAssocTypeProto, IB_FilePrivateAsso
 private class ID_PrivateAssocTypeImpl: IA_PublicAssocTypeProto, IB_FilePrivateAssocTypeProto {
   public var publicValue: Int = 0
   public var filePrivateValue: Int = 0
-  // CHECK-DAG: {{^}} internal typealias PublicValue
-  // CHECK-DAG: {{^}} internal typealias FilePrivateValue
+  // CHECK-DAG: {{^}} fileprivate typealias PublicValue
+  // CHECK-DAG: {{^}} fileprivate typealias FilePrivateValue
 } // CHECK: {{^[}]}}
 
 // CHECK-LABEL: class MultipleAttributes {