Revert "swift-module-digester: diagnose added associated types as ABI breakages even they have default type."

Author: nkcsgexi

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

@@ -69,7 +69,6 @@ cake1: Struct fixedLayoutStruct has removed conformance to P1
 
 /* Protocol Requirement Change */
 cake1: AssociatedType AssociatedTypePro.T1 has removed default type Int
-cake2: AssociatedType RequiementChanges.addedTypeWithDefault has been added as a protocol requirement
 cake2: AssociatedType RequiementChanges.addedTypeWithoutDefault has been added as a protocol requirement
 cake2: Func RequiementChanges.addedFunc() has been added as a protocol requirement
 cake2: Var RequiementChanges.addedVar has been added as a protocol requirement

test/api-digester/Outputs/Cake.txt

@@ -47,7 +47,6 @@ cake1: Struct fixedLayoutStruct has removed conformance to P1
 
 /* Protocol Requirement Change */
 cake1: AssociatedType AssociatedTypePro.T1 has removed default type Int
-cake2: AssociatedType RequiementChanges.addedTypeWithDefault has been added as a protocol requirement
 cake2: AssociatedType RequiementChanges.addedTypeWithoutDefault has been added as a protocol requirement
 cake2: Func RequiementChanges.addedFunc() has been added as a protocol requirement
 cake2: Var RequiementChanges.addedVar has been added as a protocol requirement

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

@@ -942,7 +942,14 @@ class PrunePass : public MatchedNodeListener, public SDKTreeDiffPass {
       }
       // Complain about added protocol requirements
       if (auto *D = dyn_cast<SDKNodeDecl>(Right)) {
-        if (D->isProtocolRequirement() && !D->isOverriding()) {
+        if (D->isProtocolRequirement()) {
+          bool ShouldComplain = !D->isOverriding();
+          // We should allow added associated types with default.
+          if (auto ATD = dyn_cast<SDKNodeDeclAssociatedType>(D)) {
+            if (ATD->getDefault())
+              ShouldComplain = false;
+          }
+          if (ShouldComplain)
             Ctx.getDiags().diagnose(SourceLoc(), diag::protocol_req_added,
                                     D->getScreenInfo());
         }