Index: clang/lib/Sema/SemaTemplateVariadic.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateVariadic.cpp
+++ clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -1220,10 +1220,11 @@
   if (!LHS || !RHS) {
     Expr *Pack = LHS ? LHS : RHS;
     assert(Pack && "fold expression with neither LHS nor RHS");
-    DiscardOperands();
-    if (!Pack->containsUnexpandedParameterPack())
+    if (!Pack->containsUnexpandedParameterPack()) {
+      DiscardOperands();
       return Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
              << Pack->getSourceRange();
+    }
   }
 
   BinaryOperatorKind Opc = ConvertTokenKindToBinaryOpcode(Operator);
Index: clang/test/SemaCXX/fold_expr_typo.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/fold_expr_typo.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+
+template <typename... T>
+void foo(T &&...Params) {
+  foo<T>(Unknown); // expected-error {{expression contains unexpanded parameter pack 'T'}}\
+                      expected-error {{use of undeclared identifier 'Unknown'}}
+  ((foo<T>(Unknown)), ...); // expected-error {{use of undeclared identifier 'Unknown'}}
+}
+
+template <typename... U> struct A {
+  template <typename... T> void foo(T &&...Params) {
+    foo<T>((... + static_cast<U>(1))); // expected-error {{expression contains unexpanded parameter pack 'T'}}
+  }
+};