swift-module-digester: diagnose removing default associated type as API breakage.

include/swift/AST/DiagnosticsModuleDiffer.def

@@ -62,6 +62,8 @@ ERROR(conformance_removed,none,"%0 has removed %select{conformance to|inherited
 
 ERROR(conformance_added,none,"%0 has added inherited protocol %1", (StringRef, StringRef))
 
+ERROR(default_associated_type_removed,none,"%0 has removed default type %1", (StringRef, StringRef))
+
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
 #  undef DIAG

test/api-digester/Inputs/cake1.swift

@@ -69,3 +69,8 @@ public class C7 {
 public protocol P3: P2, P1 {}
 
 extension fixedLayoutStruct: P1 {}
+
+public protocol AssociatedTypePro {
+  associatedtype T1 = Int
+  associatedtype T2
+}

test/api-digester/Inputs/cake2.swift

@@ -76,3 +76,8 @@ public protocol P3: P1, P4 {}
 public protocol P4 {}
 
 extension fixedLayoutStruct: P2 {}
+
+public protocol AssociatedTypePro {
+  associatedtype T1
+  associatedtype T2
+}

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

@@ -47,3 +47,6 @@ cake2: Var fixedLayoutStruct.lazy_d.storage is added to a non-resilient type
 cake1: Protocol P3 has added inherited protocol P4
 cake1: Protocol P3 has removed inherited protocol P2
 cake1: Struct fixedLayoutStruct has removed conformance to P1
+
+/* Protocol Requirement Change */
+cake1: AssociatedType AssociatedTypePro.T1 has removed default type Int

test/api-digester/Outputs/Cake.txt

@@ -33,3 +33,6 @@ cake1: Var C1.CIIns2 changes from strong to weak
 cake1: Protocol P3 has added inherited protocol P4
 cake1: Protocol P3 has removed inherited protocol P2
 cake1: Struct fixedLayoutStruct has removed conformance to P1
+
+/* Protocol Requirement Change */
+cake1: AssociatedType AssociatedTypePro.T1 has removed default type Int

tools/swift-api-digester/ModuleDiagsConsumer.cpp

@@ -54,6 +54,8 @@ static StringRef getCategoryName(uint32_t ID) {
   case LocalDiagID::conformance_added:
   case LocalDiagID::conformance_removed:
     return "/* Protocol Conformance Change */";
+  case LocalDiagID::default_associated_type_removed:
+    return "/* Protocol Requirement Change */";
   default:
     return StringRef();
   }

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

@@ -817,6 +817,13 @@ public:
     case NodeMatchReason::Removed:
       assert(!Right);
       Left->annotate(NodeAnnotation::Removed);
+      if (auto *LT = dyn_cast<SDKNodeType>(Left)) {
+        if (auto *AT = dyn_cast<SDKNodeDeclAssociatedType>(LT->getParent())) {
+          Ctx.getDiags().diagnose(SourceLoc(),
+                                  diag::default_associated_type_removed,
+                                  AT->getScreenInfo(), LT->getPrintedName());
+        }
+      }
       return;
     case NodeMatchReason::FuncToProperty:
     case NodeMatchReason::ModernizeEnum: