Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2134,6 +2134,8 @@
 
 def err_alignment_not_power_of_two : Error<
   "requested alignment is not a power of 2">;
+def err_alignment_dependent_typedef_name : Error<
+  "requested alignment is dependent but declaration is not dependent">;
 
 def err_attribute_aligned_too_great : Error<
   "requested alignment must be %0 bytes or smaller">;
Index: lib/Sema/SemaDeclAttr.cpp
===================================================================
--- lib/Sema/SemaDeclAttr.cpp
+++ lib/Sema/SemaDeclAttr.cpp
@@ -2863,6 +2863,17 @@
   if (!Attr.isPackExpansion() && S.DiagnoseUnexpandedParameterPack(E))
     return;
 
+  if (E->isTypeDependent() || E->isValueDependent()) {
+    if (const auto *TND = dyn_cast<TypedefNameDecl>(D)) {
+      QualType T = TND->getUnderlyingType();
+      if (!T->isDependentType() && !T->isInstantiationDependentType()) {
+        S.Diag(Attr.getLoc(), diag::err_alignment_dependent_typedef_name)
+            << E->getSourceRange();
+        return;
+      }
+    }
+  }
+
   S.AddAlignedAttr(Attr.getRange(), D, E, Attr.getAttributeSpellingListIndex(),
                    Attr.isPackExpansion());
 }
Index: test/SemaCXX/alignof.cpp
===================================================================
--- test/SemaCXX/alignof.cpp
+++ test/SemaCXX/alignof.cpp
@@ -84,3 +84,11 @@
   static_assert(sizeof(k) == alignof(long long), "");
 }
 template void n(long long);
+
+namespace PR22042 {
+template <typename T>
+void Fun(T A) {
+  typedef int __attribute__((__aligned__(A))) T1; // expected-error {{requested alignment is dependent but declaration is not dependent}}
+  int k1[__alignof__(T1)];
+}
+}