swift-module-digester: changing super classes is API breaking.

Author: nkcsgexi

include/swift/AST/DiagnosticsModuleDiffer.def

@@ -70,6 +70,8 @@ ERROR(protocol_req_added,none,"%0 has been added as a protocol requirement", (St
 
 ERROR(super_class_removed,none,"%0 has removed its super class %1", (StringRef, StringRef))
 
+ERROR(super_class_changed,none,"%0 has changed its super class from %1 to %2", (StringRef, StringRef, StringRef))
+
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
 #  undef DIAG

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

@@ -61,4 +61,5 @@ cake2: Func RequiementChanges.addedFunc() has been added as a protocol requireme
 cake2: Var RequiementChanges.addedVar has been added as a protocol requirement
 
 /* Class Inheritance Change */
+cake1: Class C4 has changed its super class from OldType to NewType
 cake1: Class SuperClassRemoval has removed its super class C3

test/api-digester/Outputs/Cake.txt

@@ -47,4 +47,5 @@ cake2: Func RequiementChanges.addedFunc() has been added as a protocol requireme
 cake2: Var RequiementChanges.addedVar has been added as a protocol requirement
 
 /* Class Inheritance Change */
+cake1: Class C4 has changed its super class from OldType to NewType
 cake1: Class SuperClassRemoval has removed its super class C3

tools/swift-api-digester/ModuleDiagsConsumer.cpp

@@ -59,6 +59,7 @@ static StringRef getCategoryName(uint32_t ID) {
   case LocalDiagID::protocol_req_added:
     return "/* Protocol Requirement Change */";
   case LocalDiagID::super_class_removed:
+  case LocalDiagID::super_class_changed:
     return "/* Class Inheritance Change */";
   default:
     return StringRef();

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

@@ -696,10 +696,15 @@ static void diagnoseNominalTypeDeclChange(SDKNodeDeclType *L, SDKNodeDeclType *R
   }
   auto LSuperClass = L->getSuperClassName();
   auto RSuperClass = R->getSuperClassName();
-  if (!LSuperClass.empty()) {
+  if (!LSuperClass.empty() && LSuperClass != RSuperClass) {
     if (RSuperClass.empty()) {
       Diags.diagnose(SourceLoc(), diag::super_class_removed, L->getScreenInfo(),
                      LSuperClass);
+    } else {
+      // FIXME: This will be a false positive if the new subclass is a subclass
+      // of the old type.
+      Diags.diagnose(SourceLoc(), diag::super_class_changed, L->getScreenInfo(),
+                     LSuperClass, RSuperClass);
     }
   }
 }