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
@@ -4540,8 +4540,15 @@
 
   // Find expected alignment, and the actual alignment of the passed object.
   // getTypeAlignInChars requires complete types
-  if (ParamTy->isIncompleteType() || ArgTy->isIncompleteType() ||
-      ParamTy->isUndeducedType() || ArgTy->isUndeducedType())
+  auto CheckTypeOK = [](QualType Ty) {
+    if (Ty->isIncompleteType() || Ty->isUndeducedType())
+      return false;
+    if (const auto *DesugaredTy = Ty->getUnqualifiedDesugaredType())
+      if (DesugaredTy->isIncompleteType() || DesugaredTy->isUndeducedType())
+        return false;
+    return true;
+  };
+  if (!CheckTypeOK(ParamTy) || !CheckTypeOK(ArgTy))
     return;
 
   CharUnits ParamAlign = Context.getTypeAlignInChars(ParamTy);
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -19661,8 +19661,13 @@
   if (isSFINAEContext())
     return ExprError();
 
-  if (T.isNull() || !Context.getLangOpts().RecoveryASTType)
+  if (T.isNull() || T->isUndeducedType() ||
+      !Context.getLangOpts().RecoveryASTType)
     // We don't know the concrete type, fallback to dependent type.
     T = Context.DependentTy;
+  else if (const auto *U = T->getUnqualifiedDesugaredType())
+    if (U->isUndeducedType())
+      T = Context.DependentTy;
+
   return RecoveryExpr::Create(Context, T, Begin, End, SubExprs);
 }
diff --git a/clang/test/SemaCXX/cxx17-undeduced-alignment.cpp b/clang/test/SemaCXX/cxx17-undeduced-alignment.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/cxx17-undeduced-alignment.cpp
@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -std=c++17 -verify %s
+//
+// Verifies that clang no longer crashes on undeduced, sugared types.
+
+template <class T> void foo(T &t);
+template <typename T>
+void bar(T t) {
+  foo(t);
+}
+
+template <typename T = void *>
+struct S { // expected-note {{candidate}}
+  S(T t);  // expected-note {{candidate}}
+  ~S();
+};
+template <typename T> S(T t) -> S<void *>;
+
+void baz() {
+  // S(123) is undeduced, but when passed to foo() via bar() it is wrapped in
+  // SubstTemplateTypeParm, for which isUndeduced() is false.
+  bar(S(123)); // expected-error {{no matching conversion}}
+}