Allow marker protocols as existentials (but still ban casts)

There is no problem with having an existential value that involves a
marker protocol. However, since there is no runtime checking for
marker protocols, ban "as?" and "is" casts to them.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -5625,8 +5625,8 @@ ERROR(marker_protocol_requirement, none,
 ERROR(marker_protocol_inherit_nonmarker, none,
       "marker protocol %0 cannot inherit non-marker protocol %1",
       (DeclName, DeclName))
-ERROR(marker_protocol_value,none,
-      "marker protocol %0 can only be used in generic constraints", (DeclName))
+ERROR(marker_protocol_cast,none,
+      "marker protocol %0 cannot be used in a conditional cast", (DeclName))
 
 //------------------------------------------------------------------------------
 // MARK: differentiable programming diagnostics

lib/Sema/MiscDiagnostics.cpp

@@ -19,6 +19,7 @@
 #include "TypeCheckConcurrency.h"
 #include "TypeChecker.h"
 #include "swift/AST/ASTWalker.h"
+#include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/NameLookupRequests.h"
 #include "swift/AST/Pattern.h"
@@ -65,6 +66,7 @@ static Expr *isImplicitPromotionToOptional(Expr *E) {
 ///   - Warn about promotions to optional in specific syntactic forms.
 ///   - Error about collection literals that default to Any collections in
 ///     invalid positions.
+///   - Marker protocols cannot occur as the type of an as? or is expression.
 ///
 static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
                                          bool isExprStmt) {
@@ -311,9 +313,31 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
         }
       }
 
+      // Diagnose checked casts that involve marker protocols.
+      if (auto cast = dyn_cast<CheckedCastExpr>(E)) {
+        checkCheckedCastExpr(cast);
+      }
+      
       return { true, E };
     }
 
+    void checkCheckedCastExpr(CheckedCastExpr *cast) {
+      if (!isa<ConditionalCheckedCastExpr>(cast) && !isa<IsExpr>(cast))
+        return;
+
+      Type castType = cast->getCastType();
+      if (!castType || !castType->isExistentialType())
+        return;
+
+      auto layout = castType->getExistentialLayout();
+      for (auto proto : layout.getProtocols()) {
+        if (proto->getDecl()->isMarkerProtocol()) {
+          Ctx.Diags.diagnose(cast->getLoc(), diag::marker_protocol_cast,
+                             proto->getDecl()->getName());
+        }
+      }
+    }
+    
     /// Visit the argument/s represented by either a ParenExpr or TupleExpr,
     /// unshuffling if needed. If any other kind of expression, will pass it
     /// straight back.

lib/Sema/TypeCheckType.cpp

@@ -3906,12 +3906,6 @@ class UnsupportedProtocolVisitor
                            proto->getName());
         T->setInvalid();
       }
-      if (proto->isMarkerProtocol()) {
-        Ctx.Diags.diagnose(comp->getNameLoc(),
-                           diag::marker_protocol_value,
-                           proto->getName());
-        T->setInvalid();
-      }
     } else if (auto *alias = dyn_cast_or_null<TypeAliasDecl>(comp->getBoundDecl())) {
       auto type = Type(alias->getDeclaredInterfaceType()->getDesugaredType());
       type.findIf([&](Type type) -> bool {
@@ -3921,15 +3915,6 @@ class UnsupportedProtocolVisitor
           auto layout = type->getExistentialLayout();
           for (auto *proto : layout.getProtocols()) {
             auto *protoDecl = proto->getDecl();
-
-            if (protoDecl->isMarkerProtocol()) {
-              Ctx.Diags.diagnose(comp->getNameLoc(),
-                                 diag::marker_protocol_value,
-                                 protoDecl->getName());
-              T->setInvalid();
-              continue;
-            }
-
             if (protoDecl->existentialTypeSupported())
               continue;
 

test/attr/attr_marker_protocol.swift

@@ -38,14 +38,14 @@ func testGenericOk(i: Int, arr: [Int], nope: [Double]) {
 
 // Incorrect uses of marker protocols in types.
 func testNotOkay(a: Any) {
-  var mp1: P3 = 17 // expected-error{{marker protocol 'P3' can only be used in generic constraints}}
+  var mp1: P3 = 17
   _ = mp1
   mp1 = 17
 
-  if let mp2 = a as? P3 { _ = mp2 } // expected-error{{marker protocol 'P3' can only be used in generic constraints}}
-  if let mp3 = a as? AnyObject & P3 { _ = mp3 } // expected-error{{marker protocol 'P3' can only be used in generic constraints}}
-  if a is AnyObject & P3 { } // expected-error{{marker protocol 'P3' can only be used in generic constraints}}
+  if let mp2 = a as? P3 { _ = mp2 } // expected-error{{marker protocol 'P3' cannot be used in a conditional cast}}
+  if let mp3 = a as? AnyObject & P3 { _ = mp3 } // expected-error{{marker protocol 'P3' cannot be used in a conditional cast}}
+  if a is AnyObject & P3 { } // expected-error{{marker protocol 'P3' cannot be used in a conditional cast}}
 
-  func inner(p3: P3) { } // expected-error{{marker protocol 'P3' can only be used in generic constraints}}
+  func inner(p3: P3) { }
 }