diff --git a/go/analysis/passes/stdversion/stdversion_test.go b/go/analysis/passes/stdversion/stdversion_test.go
index 9186527..d6a2e45 100644
--- a/go/analysis/passes/stdversion/stdversion_test.go
+++ b/go/analysis/passes/stdversion/stdversion_test.go
@@ -23,5 +23,6 @@
 	analysistest.Run(t, dir, stdversion.Analyzer,
 		"example.com/a",
 		"example.com/sub",
+		"example.com/sub20",
 		"example.com/old")
 }
diff --git a/go/analysis/passes/stdversion/testdata/test.txtar b/go/analysis/passes/stdversion/testdata/test.txtar
index 9e66cee..0d27f11 100644
--- a/go/analysis/passes/stdversion/testdata/test.txtar
+++ b/go/analysis/passes/stdversion/testdata/test.txtar
@@ -13,6 +13,7 @@
 
 use .
 use sub
+use sub20
 use old
 
 -- go.mod --
@@ -106,3 +107,42 @@
 import "go/types"
 
 var _ types.Alias // no diagnostic: go.mod is too old for us to care
+
+-- sub/oldtagged.go --
+// The file Go version (1.16) overrides the go.mod Go version (1.21),
+// even when this means a downgrade (#67123).
+// (stdversion is silent for go.mod versions before 1.21:
+// before the forward compatibility regime, the meaning
+// of the go.mod version was not clearly defined.)
+
+//go:build go1.16
+
+package sub
+
+import "bytes"
+import "go/types"
+
+var _ = bytes.Clone // want `bytes.Clone requires go1.20 or later \(file is go1.16\)`
+var _ = types.Alias // want `types.Alias requires go1.22 or later \(file is go1.16\)`
+
+-- sub20/go.mod --
+module example.com/sub20
+
+go 1.20
+
+-- sub20/oldtagged.go --
+// Same test again, but with a go1.20 mod,
+// before the forward compatibility regime:
+// The file's build tag effects selection, but
+// not language semantics, so stdversion is silent.
+
+//go:build go1.16
+
+package sub
+
+import "bytes"
+import "go/types"
+
+var _ = bytes.Clone
+var _ = types.Alias
+