diff --git a/misc/cgo/test/test.go b/misc/cgo/test/test.go
index 8c69ad91ac7..35bc3a14475 100644
--- a/misc/cgo/test/test.go
+++ b/misc/cgo/test/test.go
@@ -901,6 +901,12 @@ typedef struct S32579 { unsigned char data[1]; } S32579;
 // issue 38649
 // Test that #define'd type aliases work.
 #define netbsd_gid unsigned int
+
+// issue 40494
+// Inconsistent handling of tagged enum and union types.
+enum Enum40494 { X_40494 };
+union Union40494 { int x; };
+void issue40494(enum Enum40494 e, union Union40494* up) {}
 */
 import "C"
 
@@ -2204,3 +2210,10 @@ var issue38649 C.netbsd_gid = 42
 // issue 39877
 
 var issue39877 *C.void = nil
+
+// issue 40494
+// No runtime test; just make sure it compiles.
+
+func Issue40494() {
+	C.issue40494(C.enum_Enum40494(C.X_40494), (*C.union_Union40494)(nil))
+}
diff --git a/src/cmd/cgo/out.go b/src/cmd/cgo/out.go
index 6c221473e08..4064f0ae418 100644
--- a/src/cmd/cgo/out.go
+++ b/src/cmd/cgo/out.go
@@ -123,7 +123,9 @@ func (p *Package) writeDefs() {
 		// Moreover, empty file name makes compile emit no source debug info at all.
 		var buf bytes.Buffer
 		noSourceConf.Fprint(&buf, fset, def.Go)
-		if bytes.HasPrefix(buf.Bytes(), []byte("_Ctype_")) {
+		if bytes.HasPrefix(buf.Bytes(), []byte("_Ctype_")) ||
+			strings.HasPrefix(name, "_Ctype_enum_") ||
+			strings.HasPrefix(name, "_Ctype_union_") {
 			// This typedef is of the form `typedef a b` and should be an alias.
 			fmt.Fprintf(fgo2, "= ")
 		}