ABI checker: adding protocol requirement with default implementation should not be flagged as breakage

lib/DriverTool/swift_api_digester_main.cpp

@@ -558,6 +558,11 @@ static void diagnoseRemovedDecl(const SDKNodeDecl *D) {
   if (Ctx.getOpts().SkipRemoveDeprecatedCheck &&
       D->isDeprecated())
     return;
+  // Don't complain about removing importation of SwiftOnoneSupport.
+  if (D->getKind() == SDKNodeKind::DeclImport &&
+      D->getName() == "SwiftOnoneSupport") {
+    return;
+  }
   D->emitDiag(SourceLoc(), diag::removed_decl, false);
 }
 
@@ -691,6 +696,21 @@ class PrunePass : public MatchedNodeListener, public SDKTreeDiffPass {
             // to the allowlist.
             ShouldComplain = false;
           }
+          if (ShouldComplain) {
+            // Providing a default implementation via a protocol extension for
+            // a protocol requirement is both ABI and API safe.
+            if (auto *PD = dyn_cast<SDKNodeDecl>(D->getParent())) {
+              for (auto *SIB: PD->getChildren()) {
+                if (auto *SIBD = dyn_cast<SDKNodeDecl>(SIB)) {
+                  if (SIBD->isFromExtension() &&
+                      SIBD->getPrintedName() == D->getPrintedName()) {
+                    ShouldComplain = false;
+                    break;
+                  }
+                }
+              }
+            }
+          }
           if (ShouldComplain)
             D->emitDiag(D->getLoc(), diag::protocol_req_added);
         }

test/api-digester/protocol-req-with-default-impl.swift

@@ -0,0 +1,18 @@
+// RUN: %empty-directory(%t)
+
+// RUN: echo "public protocol P {}" > %t/Foo-1.swift
+// RUN: echo "public protocol P { func f() }" > %t/Foo-2.swift
+// RUN: echo "public extension P { func f() {} }" >> %t/Foo-2.swift
+
+// RUN: %target-swift-frontend -emit-module %t/Foo-1.swift -module-name Foo -emit-module-interface-path %t/Foo1.swiftinterface
+// RUN: %target-swift-frontend -emit-module %t/Foo-2.swift -module-name Foo -emit-module-interface-path %t/Foo2.swiftinterface
+
+// RUN: %target-swift-frontend -compile-module-from-interface %t/Foo1.swiftinterface -o %t/Foo1.swiftmodule -module-name Foo -emit-abi-descriptor-path %t/Foo1.json
+
+// RUN: %target-swift-frontend -compile-module-from-interface %t/Foo2.swiftinterface -o %t/Foo2.swiftmodule -module-name Foo -emit-abi-descriptor-path %t/Foo2.json
+
+// RUN: %api-digester -diagnose-sdk -print-module --input-paths %t/Foo1.json -input-paths %t/Foo2.json -abi -o %t/result.txt
+
+// RUN: %FileCheck %s < %t/result.txt
+
+// CHECK-NOT: added as a protocol requirement