diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -2530,12 +2530,13 @@
       // If we should expand this pack expansion now, do so.
       if (ShouldExpand) {
         for (unsigned I = 0; I != *NumExpansions; ++I) {
+          {
             Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this, I);
 
-          TypeSourceInfo *BaseTypeLoc = SubstType(Base.getTypeSourceInfo(),
-                                                  TemplateArgs,
-                                              Base.getSourceRange().getBegin(),
-                                                  DeclarationName());
+            BaseTypeLoc =
+                SubstType(Base.getTypeSourceInfo(), TemplateArgs,
+                          Base.getSourceRange().getBegin(), DeclarationName());
+          }
           if (!BaseTypeLoc) {
             Invalid = true;
             continue;
diff --git a/clang/test/SemaCXX/template-base-class-pack-expansion.cpp b/clang/test/SemaCXX/template-base-class-pack-expansion.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/template-base-class-pack-expansion.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+// Don't crash (#53609).
+
+// expected-no-diagnostics
+
+template <class, int> struct a;
+
+template <class, class...> struct b;
+template <class x, class... y, y... z>
+struct b<x, a<y, z>...> {};
+
+template <class... x> struct c: b<x>...  {};
+
+c<int> d;