[SE-0316] Ban "class subscript" in actors, too

Author: DougGregor

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2003,6 +2003,14 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       }
     }
 
+    // Reject "class" methods on actors.
+    if (SD->getStaticSpelling() == StaticSpellingKind::KeywordClass &&
+        SD->getDeclContext()->getSelfClassDecl() &&
+        SD->getDeclContext()->getSelfClassDecl()->isActor()) {
+      SD->diagnose(diag::class_subscript_not_in_class, false)
+          .fixItReplace(SD->getStaticLoc(), "static");
+    }
+
     // Now check all the accessors.
     SD->visitEmittedAccessors([&](AccessorDecl *accessor) {
       visit(accessor);

test/decl/class/actor/basic.swift

@@ -44,4 +44,7 @@ actor A2 {
 extension A2 {
   class func h2() { } // expected-error{{class methods are only allowed within classes; use 'static' to declare a static method}}
   static func i2() { } // okay
+
+  class subscript(i: Int) -> Int { i } // expected-error{{class subscripts are only allowed within classes; use 'static' to declare a static subscript}}
+  static subscript(s: String) -> String { s }
 }