diff --git a/src/sync/map.go b/src/sync/map.go
index 33bc8141ab..4f1395110a 100644
--- a/src/sync/map.go
+++ b/src/sync/map.go
@@ -36,6 +36,8 @@ import (
 //
 // [the Go memory model]: https://go.dev/ref/mem
 type Map struct {
+	_ noCopy
+
 	mu Mutex
 
 	// read contains the portion of the map's contents that are safe for
diff --git a/src/sync/mutex.go b/src/sync/mutex.go
index e4ed47c75c..cd50fcbbb5 100644
--- a/src/sync/mutex.go
+++ b/src/sync/mutex.go
@@ -34,6 +34,8 @@ func fatal(string)
 //
 // [the Go memory model]: https://go.dev/ref/mem
 type Mutex struct {
+	_ noCopy
+
 	state int32
 	sema  uint32
 }
diff --git a/src/sync/once.go b/src/sync/once.go
index 168c7bbdd3..90840b19b5 100644
--- a/src/sync/once.go
+++ b/src/sync/once.go
@@ -18,6 +18,8 @@ import (
 //
 // [the Go memory model]: https://go.dev/ref/mem
 type Once struct {
+	_ noCopy
+
 	// done indicates whether the action has been performed.
 	// It is first in the struct because it is used in the hot path.
 	// The hot path is inlined at every call site.
diff --git a/test/assign.go b/test/assign.go
index bdec58b710..5beffee6f3 100644
--- a/test/assign.go
+++ b/test/assign.go
@@ -9,7 +9,10 @@
 
 package main
 
-import "sync"
+import (
+	"sync"
+	"time"
+)
 
 type T struct {
 	int
@@ -38,7 +41,7 @@ func main() {
 		_ = x
 	}
 	{
-		x := sync.Mutex{0, 0} // ERROR "assignment.*Mutex"
+		x := time.Time{0, 0, nil} // ERROR "assignment.*Time"
 		_ = x
 	}
 	{