diff --git a/clang-tools-extra/pseudo/lib/cxx.bnf b/clang-tools-extra/pseudo/lib/cxx.bnf
--- a/clang-tools-extra/pseudo/lib/cxx.bnf
+++ b/clang-tools-extra/pseudo/lib/cxx.bnf
@@ -429,7 +429,7 @@
 ptr-abstract-declarator := noptr-abstract-declarator
 ptr-abstract-declarator := ptr-operator ptr-abstract-declarator_opt
 noptr-abstract-declarator := noptr-abstract-declarator_opt parameters-and-qualifiers
-noptr-abstract-declarator := noptr-abstract-declarator_opt [ constant-expression ]
+noptr-abstract-declarator := noptr-abstract-declarator_opt [ constant-expression_opt ]
 noptr-abstract-declarator := ( ptr-abstract-declarator )
 abstract-pack-declarator := noptr-abstract-pack-declarator
 abstract-pack-declarator := ptr-operator abstract-pack-declarator
diff --git a/clang-tools-extra/pseudo/test/cxx/unsized-array.cpp b/clang-tools-extra/pseudo/test/cxx/unsized-array.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/pseudo/test/cxx/unsized-array.cpp
@@ -0,0 +1,7 @@
+// RUN: clang-pseudo -grammar=%cxx-bnf-file -source=%s --print-forest | FileCheck %s
+void s(int[]);
+// CHECK:      parameter-declaration-list~parameter-declaration := decl-specifier-seq abstract-declarator
+// CHECK-NEXT: ├─decl-specifier-seq~INT := tok[3]
+// CHECK-NEXT: └─abstract-declarator~noptr-abstract-declarator := [ ]
+// CHECK-NEXT:   ├─[ := tok[4]
+// CHECK-NEXT:   └─] := tok[5]