[AST] Rework diagnostic about init accessors inside of extensions

Author: xedin

include/swift/AST/DiagnosticsParse.def

@@ -2159,7 +2159,7 @@ ERROR(init_accessor_is_not_on_property,none,
       ())
 
 ERROR(init_accessor_is_not_in_the_primary_declaration,none,
-      "init accessors could only be declared in the primary declaration",
+      "init accessors cannot be declared in an extension",
       ())
 
 ERROR(missing_storage_restrictions_attr_label,none,

lib/Parse/ParseDecl.cpp

@@ -8497,12 +8497,11 @@ void Parser::ParsedAccessors::classify(Parser &P, AbstractStorageDecl *storage,
   }
 
   if (Init) {
-    if (storage->getDeclContext()->getContextKind() ==
-        DeclContextKind::ExtensionDecl) {
+    auto *DC = storage->getDeclContext();
+    if (isa<ExtensionDecl>(DC)) {
       P.diagnose(Init->getLoc(),
                  diag::init_accessor_is_not_in_the_primary_declaration);
-    } else if (!storage->getDeclContext()->getSelfNominalTypeDecl() ||
-               isa<SubscriptDecl>(storage)) {
+    } else if (!DC->isTypeContext() || isa<SubscriptDecl>(storage)) {
       P.diagnose(Init->getLoc(), diag::init_accessor_is_not_on_property);
     }
   }

test/decl/var/init_accessors.swift

@@ -699,7 +699,7 @@ extension Test2ForExtension {
   var extendedX: Int {
     @storageRestrictions(initializes: _x)
     init {
-    // expected-error@-1 {{init accessors could only be declared in the primary declaration}}
+      // expected-error@-1 {{init accessors cannot be declared in an extension}}
       self._x = newValue
     }
     get { _x }