Index: clang/lib/Sema/SemaTemplate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplate.cpp
+++ clang/lib/Sema/SemaTemplate.cpp
@@ -1935,6 +1935,8 @@
   // FIXME: Add a kind for this to give more meaningful diagnostics. But can
   // this substitution process actually fail?
   InstantiatingTemplate BuildingDeductionGuides(*this, Loc, Template);
+  if (BuildingDeductionGuides.isInvalid())
+    return;
 
   // Convert declared constructors into deduction guide templates.
   // FIXME: Skip constructors for which deduction must necessarily fail (those
Index: clang/test/SemaTemplate/instantiate-after-fatal-cxx17.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaTemplate/instantiate-after-fatal-cxx17.cpp
@@ -0,0 +1,16 @@
+// RUN: not %clang_cc1 -std=c++17 -fsyntax-only -ferror-limit 1 %s 2>&1 | FileCheck %s
+
+#error Error 1
+#error Error 2
+// CHECK: fatal error: too many errors emitted, stopping now
+
+namespace rdar39051732 {
+
+  template<class T> struct A {
+    template <class U> A(T&, ...);
+  };
+  // Deduction guide triggers constructor instantiation.
+  template<class T> A(const T&, const T&) -> A<T&>;
+
+}
+