# UNDF: UNDF-2026-000000825
diff --git a/src/cmd/compile/internal/types2/infer.go b/src/cmd/compile/internal/types2/infer.go
index eeefb117..cwe407fix 100644
--- a/src/cmd/compile/internal/types2/infer.go
+++ b/src/cmd/compile/internal/types2/infer.go
@@ -543,14 +543,20 @@ func isParameterized(tparams []*TypeParam, typ Type) bool {
-	w := tpWalker{
-		tparams: tparams,
-		seen:    make(map[Type]bool),
-	}
+	// CWE-407 fix: pre-build a map for O(1) TypeParam membership check.
+	// Previously tpWalker used a []*TypeParam slice and called slices.Index
+	// (O(n)) at every *TypeParam node during the walk — O(n²) total for n
+	// type parameters in a generic function.
+	tset := make(map[*TypeParam]bool, len(tparams))
+	for _, tp := range tparams {
+		tset[tp] = true
+	}
+	w := tpWalker{
+		tparams: tset,
+		seen:    make(map[Type]bool),
+	}
 	return w.isParameterized(typ)
 }

 type tpWalker struct {
-	tparams []*TypeParam
+	tparams map[*TypeParam]bool  // CWE-407: was []*TypeParam (O(n) contains)
 	seen    map[Type]bool
 }

@@ -627,7 +633,7 @@ func (w *tpWalker) isParameterized(typ Type) (res bool) {

 	case *TypeParam:
-		return slices.Index(w.tparams, t) >= 0
+		return w.tparams[t]  // CWE-407: O(1) map lookup, was O(n) slices.Index

 	default:
 		panic(fmt.Sprintf("unexpected %T", typ))