diff --git a/include/swift/AST/Concurrency.h b/include/swift/AST/Concurrency.h
index 3d19fa40bbe80..c5e0de233c4b4 100644
--- a/include/swift/AST/Concurrency.h
+++ b/include/swift/AST/Concurrency.h
@@ -19,6 +19,11 @@
 
 namespace swift {
 
+/// Find the imported module that treats the given nominal type as "preconcurrency", or return `nullptr`
+/// if there is no such module.
+ModuleDecl *moduleImportForPreconcurrency(NominalTypeDecl *nominal,
+                                          const DeclContext *fromDC);
+
 /// Determinate the appropriate diagnostic behavior to used when emitting
 /// concurrency diagnostics when referencing the given nominal type from the
 /// given declaration context.
diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 537f65b69a202..9a5ef27378953 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -8681,9 +8681,9 @@ GROUPED_ERROR(isolated_conformance_wrong_domain,IsolatedConformances,none,
 GROUPED_WARNING(isolated_conformance_will_become_nonisolated,IsolatedConformances,none,
       "conformance of %0 to %1 should be marked 'nonisolated' to retain its behavior with upcoming feature 'InferIsolatedConformances'",
       (const ValueDecl *, const ValueDecl *))
-GROUPED_WARNING(isolated_conformance_to_sendable_metatype,IsolatedConformances,none,
-    "%0 conformance of %1 to SendableMetatype-inheriting %kind2 can never "
-    "be used with generic code", (ActorIsolation, Type, const ValueDecl *))
+GROUPED_ERROR(isolated_conformance_to_sendable_metatype,IsolatedConformances,none,
+    "cannot form %0 conformance of %1 to SendableMetatype-inheriting %kind2",
+    (ActorIsolation, Type, const ValueDecl *))
 
 //===----------------------------------------------------------------------===//
 // MARK: @_inheritActorContext
diff --git a/lib/AST/Concurrency.cpp b/lib/AST/Concurrency.cpp
index 958ac706c692e..c2929dcf2c0dc 100644
--- a/lib/AST/Concurrency.cpp
+++ b/lib/AST/Concurrency.cpp
@@ -19,29 +19,35 @@
 
 using namespace swift;
 
-std::optional<DiagnosticBehavior>
-swift::getConcurrencyDiagnosticBehaviorLimit(NominalTypeDecl *nominal,
-                                             const DeclContext *fromDC,
-                                             bool ignoreExplicitConformance) {
-  ModuleDecl *importedModule = nullptr;
+ModuleDecl *swift::moduleImportForPreconcurrency(
+    NominalTypeDecl *nominal, const DeclContext *fromDC) {
+  // If the declaration itself has the @preconcurrency attribute,
+  // respect it.
   if (nominal->getAttrs().hasAttribute<PreconcurrencyAttr>()) {
-    // If the declaration itself has the @preconcurrency attribute,
-    // respect it.
-    importedModule = nominal->getParentModule();
-  } else {
-    // Determine whether this nominal type is visible via a @preconcurrency
-    // import.
-    auto import = nominal->findImport(fromDC);
-    auto sourceFile = fromDC->getParentSourceFile();
+    return nominal->getParentModule();
+  }
 
-    if (!import || !import->options.contains(ImportFlags::Preconcurrency))
-      return std::nullopt;
+  // Determine whether this nominal type is visible via a @preconcurrency
+  // import.
+  auto import = nominal->findImport(fromDC);
+  auto sourceFile = fromDC->getParentSourceFile();
 
-    if (sourceFile)
-      sourceFile->setImportUsedPreconcurrency(*import);
+  if (!import || !import->options.contains(ImportFlags::Preconcurrency))
+    return nullptr;
 
-    importedModule = import->module.importedModule;
-  }
+  if (sourceFile)
+    sourceFile->setImportUsedPreconcurrency(*import);
+
+  return import->module.importedModule;
+}
+
+std::optional<DiagnosticBehavior>
+swift::getConcurrencyDiagnosticBehaviorLimit(NominalTypeDecl *nominal,
+                                             const DeclContext *fromDC,
+                                             bool ignoreExplicitConformance) {
+  ModuleDecl *importedModule = moduleImportForPreconcurrency(nominal, fromDC);
+  if (!importedModule)
+    return std::nullopt;
 
   // When the type is explicitly non-Sendable, @preconcurrency imports
   // downgrade the diagnostic to a warning in Swift 6.
diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index af6f0f4d9d7ba..beb691c5830cb 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -8303,18 +8303,20 @@ RawConformanceIsolationRequest::evaluate(
       globalActorTypeExpr->setType(MetatypeType::get(globalActorType));
     }
 
-    // Isolated conformance to a SendableMetatype-inheriting protocol can
-    // never be used generically. Warn about it.
+    // Cannot form an isolated conformance to a SendableMetatype-inheriting
+    // protocol. Diagnose it.
     if (auto sendableMetatype =
             ctx.getProtocol(KnownProtocolKind::SendableMetatype)) {
-      if (proto->inheritsFrom(sendableMetatype) &&
-          !getActorIsolation(proto).preconcurrency()) {
+      if (proto->inheritsFrom(sendableMetatype)) {
+        bool isPreconcurrency = moduleImportForPreconcurrency(
+            proto, conformance->getDeclContext()) != nullptr;
         ctx.Diags.diagnose(
             conformance->getLoc(),
             diag::isolated_conformance_to_sendable_metatype,
             ActorIsolation::forGlobalActor(globalActorType),
             conformance->getType(),
-            proto);
+            proto)
+          .limitBehaviorIf(isPreconcurrency, DiagnosticBehavior::Warning);
       }
     }
 
diff --git a/test/Concurrency/isolated_conformance.swift b/test/Concurrency/isolated_conformance.swift
index 3b1ef6430f70a..8f71f601b24be 100644
--- a/test/Concurrency/isolated_conformance.swift
+++ b/test/Concurrency/isolated_conformance.swift
@@ -151,7 +151,7 @@ protocol R: SendableMetatype {
   func f()
 }
 
-// expected-warning@+1{{main actor-isolated conformance of 'RSendableSMainActor' to SendableMetatype-inheriting protocol 'R' can never be used with generic code}}
+// expected-error@+1{{cannot form main actor-isolated conformance of 'RSendableSMainActor' to SendableMetatype-inheriting protocol 'R'}}
 @MainActor struct RSendableSMainActor: @MainActor R {
   func f() { }
 }
diff --git a/test/Concurrency/isolated_conformance_6.swift b/test/Concurrency/isolated_conformance_6.swift
new file mode 100644
index 0000000000000..c77dad3f1520b
--- /dev/null
+++ b/test/Concurrency/isolated_conformance_6.swift
@@ -0,0 +1,22 @@
+// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 6 %s
+
+// REQUIRES: concurrency
+
+protocol P: SendableMetatype {
+  func f()
+}
+
+@preconcurrency
+protocol Q: SendableMetatype {
+  func f()
+}
+
+// expected-error@+1{{cannot form main actor-isolated conformance of 'PSendableSMainActor' to SendableMetatype-inheriting protocol 'P'}}
+@MainActor struct PSendableSMainActor: @MainActor P {
+  func f() { }
+}
+
+// expected-warning@+1{{cannot form main actor-isolated conformance of 'QSendableSMainActor' to SendableMetatype-inheriting protocol 'Q'}}
+@MainActor struct QSendableSMainActor: @MainActor Q {
+  func f() { }
+}