diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -4492,7 +4492,8 @@
 
   // Find expected alignment, and the actual alignment of the passed object.
   // getTypeAlignInChars requires complete types
-  if (ParamTy->isIncompleteType() || ArgTy->isIncompleteType())
+  if (ParamTy->isIncompleteType() || ArgTy->isIncompleteType() ||
+      ParamTy->isUndeducedType() || ArgTy->isUndeducedType())
     return;
 
   CharUnits ParamAlign = Context.getTypeAlignInChars(ParamTy);
diff --git a/clang/test/Sema/check-arg-alignment.cpp b/clang/test/Sema/check-arg-alignment.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/check-arg-alignment.cpp
@@ -0,0 +1,13 @@
+// Ensure we don't crash when CTAD fails.
+// RUN: %clang_cc1 -std=c++17 -verify %s
+template <typename T1, typename T2>
+struct Foo {   // expected-note{{candidate function template not viable}}
+  Foo(T1, T2); // expected-note{{candidate function template not viable}}
+};
+
+template <typename... Args>
+void insert(Args &&...args);
+
+void foo() {
+  insert(Foo(2, 2, 2)); // expected-error{{no viable constructor or deduction guide}}
+}