diff --git a/include/swift/AST/DiagnosticsModuleDiffer.def b/include/swift/AST/DiagnosticsModuleDiffer.def
index 443117a8d6afa..b8d7276a150d5 100644
--- a/include/swift/AST/DiagnosticsModuleDiffer.def
+++ b/include/swift/AST/DiagnosticsModuleDiffer.def
@@ -88,6 +88,8 @@ ERROR(not_inheriting_convenience_inits,APIDigesterBreakage,"%0 no longer inherit
 
 ERROR(enum_case_added,APIDigesterBreakage,"%0 has been added as a new enum case", (StringRef))
 
+ERROR(demangled_name_changed,APIDigesterBreakage,"%0 has mangled name changing from '%1' to '%2'", (StringRef, StringRef, StringRef))
+
 WARNING(cannot_read_allowlist,none,"cannot read breakage allowlist at '%0'", (StringRef))
 
 #define UNDEFINE_DIAGNOSTIC_MACROS
diff --git a/include/swift/AST/USRGeneration.h b/include/swift/AST/USRGeneration.h
index 7b96c4d3e3c27..4d4f116c0154c 100644
--- a/include/swift/AST/USRGeneration.h
+++ b/include/swift/AST/USRGeneration.h
@@ -61,6 +61,9 @@ bool printExtensionUSR(const ExtensionDecl *ED, raw_ostream &OS);
 /// \returns true if it failed, false on success.
 bool printDeclUSR(const Decl *D, raw_ostream &OS);
 
+/// Demangle a mangle-name-based USR to a human readable name.
+std::string demangleUSR(StringRef mangled);
+
 } // namespace ide
 } // namespace swift
 
diff --git a/lib/APIDigester/ModuleAnalyzerNodes.cpp b/lib/APIDigester/ModuleAnalyzerNodes.cpp
index 81f20ca5cc880..eaebc11e43bc4 100644
--- a/lib/APIDigester/ModuleAnalyzerNodes.cpp
+++ b/lib/APIDigester/ModuleAnalyzerNodes.cpp
@@ -944,6 +944,8 @@ static bool isSDKNodeEqual(SDKContext &Ctx, const SDKNode &L, const SDKNode &R)
         if (Left->getFixedBinaryOrder() != Right->getFixedBinaryOrder())
           return false;
       }
+      if (Left->getUsr() != Right->getUsr())
+        return false;
       LLVM_FALLTHROUGH;
     }
     case SDKNodeKind::Conformance:
@@ -2551,6 +2553,13 @@ void swift::ide::api::SDKNodeDecl::diagnose(SDKNode *Right) {
       emitDiag(Loc, diag::decl_reorder, getFixedBinaryOrder(),
                RD->getFixedBinaryOrder());
     }
+    if (getUsr() != RD->getUsr()) {
+      auto left = demangleUSR(getUsr());
+      auto right = demangleUSR(RD->getUsr());
+      if (left != right) {
+        emitDiag(Loc, diag::demangled_name_changed, left, right);
+      }
+    }
   }
 }
 
diff --git a/lib/APIDigester/ModuleDiagsConsumer.cpp b/lib/APIDigester/ModuleDiagsConsumer.cpp
index 044c9d05926d9..c73fd148d5947 100644
--- a/lib/APIDigester/ModuleDiagsConsumer.cpp
+++ b/lib/APIDigester/ModuleDiagsConsumer.cpp
@@ -54,6 +54,7 @@ static StringRef getCategoryName(uint32_t ID) {
   case LocalDiagID::raw_type_change:
     return "/* RawRepresentable Changes */";
   case LocalDiagID::generic_sig_change:
+  case LocalDiagID::demangled_name_changed:
     return "/* Generic Signature Changes */";
   case LocalDiagID::enum_case_added:
   case LocalDiagID::decl_added:
diff --git a/lib/AST/USRGeneration.cpp b/lib/AST/USRGeneration.cpp
index c3fe2322c21c9..5a02175895e8f 100644
--- a/lib/AST/USRGeneration.cpp
+++ b/lib/AST/USRGeneration.cpp
@@ -19,6 +19,7 @@
 #include "swift/AST/SwiftNameTranslation.h"
 #include "swift/AST/TypeCheckRequests.h"
 #include "swift/AST/USRGeneration.h"
+#include "swift/Demangling/Demangler.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/raw_ostream.h"
@@ -255,6 +256,18 @@ swift::USRGenerationRequest::evaluate(Evaluator &evaluator,
   return NewMangler.mangleDeclAsUSR(D, getUSRSpacePrefix());
 }
 
+std::string ide::demangleUSR(StringRef mangled) {
+  if (mangled.startswith(getUSRSpacePrefix())) {
+    mangled = mangled.substr(getUSRSpacePrefix().size());
+  }
+  SmallString<128> buffer;
+  buffer += "$s";
+  buffer += mangled;
+  mangled = buffer.str();
+  Demangler Dem;
+  return nodeToString(Dem.demangleSymbol(mangled));
+}
+
 std::string
 swift::MangleLocalTypeDeclRequest::evaluate(Evaluator &evaluator,
                                             const TypeDecl *D) const {
diff --git a/test/api-digester/Outputs/Cake-abi.txt b/test/api-digester/Outputs/Cake-abi.txt
index a0ed1dcc8ac50..7010f162d7149 100644
--- a/test/api-digester/Outputs/Cake-abi.txt
+++ b/test/api-digester/Outputs/Cake-abi.txt
@@ -1,7 +1,17 @@
 
 /* Generic Signature Changes */
+cake: Constructor S1.init(_:) has mangled name changing from 'cake.S1.init(Swift.Int) -> cake.S1' to 'cake.S1.init(Swift.Double) -> cake.S1'
+cake: Func C1.foo1() has mangled name changing from 'static cake.C1.foo1() -> ()' to 'cake.C1.foo1() -> ()'
+cake: Func C1.foo2(_:) has mangled name changing from 'cake.C1.foo2(Swift.Int) -> ()' to 'cake.C1.foo2(() -> ()) -> ()'
 cake: Func P1.P1Constraint() has generic signature change from <Self where Self : cake.P1, Self : cake.P2> to <Self where Self : cake.P1>
+cake: Func P1.P1Constraint() has mangled name changing from '(extension in cake):cake.P1< where A: cake.P2>.P1Constraint() -> ()' to '(extension in cake):cake.P1.P1Constraint() -> ()'
+cake: Func S1.foo3() has mangled name changing from 'cake.S1.foo3() -> ()' to 'static cake.S1.foo3() -> ()'
+cake: Func S1.foo5(x:y:) has mangled name changing from 'cake.S1.foo5(x: Swift.Int, y: Swift.Int) -> ()' to 'cake.S1.foo5(x: Swift.Int, y: Swift.Int, z: Swift.Int) -> ()'
+cake: Func Somestruct2.foo1(_:) has mangled name changing from 'static cake.Somestruct2.foo1(cake.C3) -> ()' to 'static cake.NSSomestruct2.foo1(cake.C1) -> ()'
+cake: Func ownershipChange(_:_:) has mangled name changing from 'cake.ownershipChange(inout Swift.Int, __shared Swift.Int) -> ()' to 'cake.ownershipChange(Swift.Int, __owned Swift.Int) -> ()'
+cake: Func returnFunctionTypeOwnershipChange() has mangled name changing from 'cake.returnFunctionTypeOwnershipChange() -> (cake.C1) -> ()' to 'cake.returnFunctionTypeOwnershipChange() -> (__owned cake.C1) -> ()'
 cake: Protocol P3 has generic signature change from <Self : cake.P1, Self : cake.P2> to <Self : cake.P1, Self : cake.P4>
+cake: Struct Somestruct2 has mangled name changing from 'cake.Somestruct2' to 'cake.NSSomestruct2'
 
 /* RawRepresentable Changes */
 
diff --git a/test/api-digester/diff-demangled-name.swift b/test/api-digester/diff-demangled-name.swift
new file mode 100644
index 0000000000000..3a5d8290508d4
--- /dev/null
+++ b/test/api-digester/diff-demangled-name.swift
@@ -0,0 +1,21 @@
+// RUN: %empty-directory(%t)
+// RUN: echo "public func foo() {}" > %t/Foo.swift
+
+// RUN: echo "public protocol P { associatedtype A }" > %t/Foo-1.swift
+// RUN: echo "public extension P { public func f() where A == Int  {} }" >> %t/Foo-1.swift
+
+// RUN: echo "public protocol P { associatedtype A }" > %t/Foo-2.swift
+// RUN: echo "public extension P where A == Int { public 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: Foo: Func P.f() has mangled name changing from '(extension in Foo):Foo.P.f< where A.A == Swift.Int>() -> ()' to '(extension in Foo):Foo.P< where A.A == Swift.Int>.f() -> ()'