diff --git a/src/tint/transform/expand_compound_assignment.cc b/src/tint/transform/expand_compound_assignment.cc
index 2f775ca119..d15d790dab 100644
--- a/src/tint/transform/expand_compound_assignment.cc
+++ b/src/tint/transform/expand_compound_assignment.cc
@@ -44,6 +44,8 @@ bool ExpandCompoundAssignment::ShouldRun(const Program* program, const DataMap&)
     return false;
 }
 
+namespace {
+
 /// Internal class used to collect statement expansions during the transform.
 class State {
   private:
@@ -163,6 +165,8 @@ class State {
     }
 };
 
+}  // namespace
+
 void ExpandCompoundAssignment::Run(CloneContext& ctx, const DataMap&, DataMap&) const {
     State state(ctx);
     for (auto* node : ctx.src->ASTNodes().Objects()) {