diff --git a/lib/Sema/TypeCheckDeclPrimary.cpp b/lib/Sema/TypeCheckDeclPrimary.cpp
index 6837b674ec583..3905ba039b53d 100644
--- a/lib/Sema/TypeCheckDeclPrimary.cpp
+++ b/lib/Sema/TypeCheckDeclPrimary.cpp
@@ -1689,8 +1689,10 @@ static void diagnoseRetroactiveConformances(
               extTypeModule->isSameModuleLookingThroughOverlays(module);
           auto incorrectTypeName = typeIsSameModule ? 
               extendedNominalDecl->getName() : proto->getName();
-          diags.diagnose(loc, diag::retroactive_attr_does_not_apply, 
-              incorrectTypeName)
+          diags
+              .diagnose(loc, diag::retroactive_attr_does_not_apply,
+                        incorrectTypeName)
+              .warnUntilSwiftVersion(6)
               .fixItRemove(SourceRange(loc, loc.getAdvancedLoc(1)));
           return TypeWalker::Action::Stop;
         }
diff --git a/test/Sema/extension_retroactive_conformances.swift b/test/Sema/extension_retroactive_conformances.swift
index 04f4fe7b415e7..8a05c57413ddf 100644
--- a/test/Sema/extension_retroactive_conformances.swift
+++ b/test/Sema/extension_retroactive_conformances.swift
@@ -65,9 +65,9 @@ extension Sample1: ClientProtocol {}
 
 struct Sample7: @retroactive SampleProtocol1 {} // expected-error {{'retroactive' attribute only applies in inheritance clauses in extensions}}
 
-extension Sample7: @retroactive ClientProtocol {} // expected-error {{'retroactive' attribute does not apply; 'Sample7' is declared in this module}}
+extension Sample7: @retroactive ClientProtocol {} // expected-warning {{'retroactive' attribute does not apply; 'Sample7' is declared in this module}}
 
-extension Int: @retroactive ClientProtocol {} // expected-error {{'retroactive' attribute does not apply; 'ClientProtocol' is declared in this module}}
+extension Int: @retroactive ClientProtocol {} // expected-warning {{'retroactive' attribute does not apply; 'ClientProtocol' is declared in this module}}
 
 func f(_ x: @retroactive Int) {} // expected-error {{'retroactive' attribute only applies in inheritance clauses in extensions}}
 
diff --git a/test/Sema/extension_retroactive_conformances_swift6.swift b/test/Sema/extension_retroactive_conformances_swift6.swift
new file mode 100644
index 0000000000000..cd02e73c507de
--- /dev/null
+++ b/test/Sema/extension_retroactive_conformances_swift6.swift
@@ -0,0 +1,10 @@
+// RUN: %target-typecheck-verify-swift -swift-version 6
+
+// REQUIRES: asserts
+
+struct LocalStruct {}
+protocol LocalProtocol {}
+
+extension LocalStruct: @retroactive LocalProtocol {} // expected-error {{'retroactive' attribute does not apply; 'LocalStruct' is declared in this module}}
+
+extension Int: @retroactive LocalProtocol {} // expected-error {{'retroactive' attribute does not apply; 'LocalProtocol' is declared in this module}}