[ExistentialLayout] Introduce getKind() and use it.

Suggested by John McCall.

Author: Davide Italiano

include/swift/AST/ExistentialLayout.h

@@ -28,6 +28,8 @@ namespace swift {
   class ProtocolCompositionType;
 
 struct ExistentialLayout {
+  enum Kind { Class, Error, Opaque };
+
   ExistentialLayout() {
     hasExplicitAnyObject = false;
     containsNonObjCProtocol = false;
@@ -46,6 +48,19 @@ struct ExistentialLayout {
   /// Whether any protocol members are non-@objc.
   bool containsNonObjCProtocol : 1;
 
+  /// Return the kind of this existential (class/error/opaque).
+  Kind getKind() {
+    if (requiresClass())
+      return Kind::Class;
+    if (isErrorExistential())
+      return Kind::Error;
+
+    // The logic here is that opaque is the complement of class + error,
+    // i.e. we don't have more concrete information guiding the layout
+    // and it doesn't fall into the special-case Error representation.
+    return Kind::Opaque;
+  }
+
   bool isAnyObject() const;
 
   bool isObjC() const {

lib/SIL/SILType.cpp

@@ -356,20 +356,22 @@ SILType::canUseExistentialRepresentation(SILModule &M,
 
     auto layout = getASTType().getExistentialLayout();
 
+    switch (layout.getKind()) {
+    // A class-constrained composition uses ClassReference representation;
+    // otherwise, we use a fixed-sized buffer.
+    case ExistentialLayout::Kind::Class:
+      return repr == ExistentialRepresentation::Class;
     // The (uncomposed) Error existential uses a special boxed
-    // representation. It can also adopt class references of bridged error types
-    // directly.
-    if (layout.isErrorExistential())
+    // representation. It can also adopt class references of bridged
+    // error types directly.
+    case ExistentialLayout::Kind::Error:
       return repr == ExistentialRepresentation::Boxed
         || (repr == ExistentialRepresentation::Class
             && isBridgedErrorClass(M, containedType));
-    
-    // A class-constrained composition uses ClassReference representation;
-    // otherwise, we use a fixed-sized buffer.
-    if (layout.requiresClass())
-      return repr == ExistentialRepresentation::Class;
-
-    return repr == ExistentialRepresentation::Opaque;
+    case ExistentialLayout::Kind::Opaque:
+      return repr == ExistentialRepresentation::Opaque;
+    }
+    llvm_unreachable("unknown existential kind!");
   }
   case ExistentialRepresentation::Metatype:
     return is<ExistentialMetatypeType>();