swift-module-digester: diagnose any decl kind changes for nominal types.

include/swift/AST/DiagnosticsModuleDiffer.def

@@ -72,6 +72,8 @@ ERROR(super_class_removed,none,"%0 has removed its super class %1", (StringRef,
 
 ERROR(super_class_changed,none,"%0 has changed its super class from %1 to %2", (StringRef, StringRef, StringRef))
 
+ERROR(nominal_type_kind_changed,none,"%0 has been changed to a %1", (StringRef, StringRef))
+
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
 #  undef DIAG

test/api-digester/Inputs/cake1.swift

@@ -91,3 +91,6 @@ public protocol RequiementChanges {
 }
 
 public class SuperClassRemoval: C3 {}
+
+public class ClassToStruct {}
+public protocol ProtocolToEnum {}

test/api-digester/Inputs/cake2.swift

@@ -98,3 +98,6 @@ public protocol RequiementChanges {
 }
 
 public class SuperClassRemoval {}
+
+public struct ClassToStruct {}
+public enum ProtocolToEnum {}

test/api-digester/Outputs/Cake-abi.txt

@@ -16,6 +16,8 @@ cake1: Var RemoveSetters.Value has removed its setter
 cake1: Var RequiementChanges.removedVar has been removed
 
 /* Moved Decls */
+cake1: Class ClassToStruct has been changed to a Struct
+cake1: Protocol ProtocolToEnum has been changed to a Enum
 
 /* Renamed Decls */
 cake1: Func S1.foo5(x:y:) has been renamed to Func S1.foo5(x:y:z:)

test/api-digester/Outputs/Cake.txt

@@ -16,6 +16,8 @@ cake1: Var RemoveSetters.Value has removed its setter
 cake1: Var RequiementChanges.removedVar has been removed
 
 /* Moved Decls */
+cake1: Class ClassToStruct has been changed to a Struct
+cake1: Protocol ProtocolToEnum has been changed to a Enum
 
 /* Renamed Decls */
 cake1: Func S1.foo5(x:y:) has been renamed to Func S1.foo5(x:y:z:)

tools/swift-api-digester/ModuleAnalyzerNodes.cpp

@@ -764,6 +764,9 @@ bool SDKNode::operator==(const SDKNode &Other) const {
         if (Left->getSuperClassName() != Right->getSuperClassName()) {
           return false;
         }
+        if (Left->getDeclKind() != Right->getDeclKind()) {
+          return false;
+        }
       }
       LLVM_FALLTHROUGH;
     }

tools/swift-api-digester/ModuleDiagsConsumer.cpp

@@ -36,6 +36,7 @@ static StringRef getCategoryName(uint32_t ID) {
   case LocalDiagID::removed_setter:
     return "/* Removed Decls */";
   case LocalDiagID::moved_decl:
+  case LocalDiagID::nominal_type_kind_changed:
     return "/* Moved Decls */";
   case LocalDiagID::renamed_decl:
     return "/* Renamed Decls */";

tools/swift-api-digester/swift-api-digester.cpp

@@ -677,6 +677,15 @@ static bool isOwnershipEquivalent(ReferenceOwnership Left,
 static void diagnoseNominalTypeDeclChange(SDKNodeDeclType *L, SDKNodeDeclType *R) {
   auto &Ctx = L->getSDKContext();
   auto &Diags = Ctx.getDiags();
+
+  if (L->getDeclKind() != R->getDeclKind()) {
+    Diags.diagnose(SourceLoc(), diag::nominal_type_kind_changed,
+      L->getScreenInfo(), getDeclKindStr(R->getDeclKind()));
+    return;
+  }
+
+  assert(L->getDeclKind() == R->getDeclKind());
+  auto DKind = L->getDeclKind();
   std::vector<StringRef> LeftMinusRight;
   std::vector<StringRef> RightMinusLeft;
   swift::ide::api::stringSetDifference(L->getAllProtocols(), R->getAllProtocols(),
@@ -686,14 +695,15 @@ static void diagnoseNominalTypeDeclChange(SDKNodeDeclType *L, SDKNodeDeclType *R
     Diags.diagnose(SourceLoc(), diag::conformance_removed, L->getScreenInfo(), Name,
                    isProtocol);
   });
-
-  // Adding inherited protocols can be API breaking.
-  if (isProtocol) {
+  switch (DKind) {
+  case DeclKind::Protocol: {
     std::for_each(RightMinusLeft.begin(), RightMinusLeft.end(), [&](StringRef Name) {
       Diags.diagnose(SourceLoc(), diag::conformance_added, L->getScreenInfo(),
                      Name);
     });
+    break;
   }
+  case DeclKind::Class: {
     auto LSuperClass = L->getSuperClassName();
     auto RSuperClass = R->getSuperClassName();
     if (!LSuperClass.empty() && LSuperClass != RSuperClass) {
@@ -707,6 +717,11 @@ static void diagnoseNominalTypeDeclChange(SDKNodeDeclType *L, SDKNodeDeclType *R
                        LSuperClass, RSuperClass);
       }
     }
+    break;
+  }
+  default:
+    break;
+  }
 }
 
 static void detectDeclChange(NodePtr L, NodePtr R, SDKContext &Ctx) {