diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -5346,11 +5346,11 @@
 QualType ASTContext::getDecltypeType(Expr *e, QualType UnderlyingType) const {
   DecltypeType *dt;
 
-  // C++11 [temp.type]p2:
-  //   If an expression e involves a template parameter, decltype(e) denotes a
+  // C++17 [temp.type]p2:
+  //   If an expression e is type-dependent (17.6.2.2), decltype(e) denotes a
   //   unique dependent type. Two such decltype-specifiers refer to the same
-  //   type only if their expressions are equivalent (14.5.6.1).
-  if (e->isInstantiationDependent()) {
+  //   type only if their expressions are equivalent (17.5.6.1).
+  if (e->isTypeDependent()) {
     llvm::FoldingSetNodeID ID;
     DependentDecltypeType::Profile(ID, *this, e);
 
diff --git a/clang/lib/AST/Type.cpp b/clang/lib/AST/Type.cpp
--- a/clang/lib/AST/Type.cpp
+++ b/clang/lib/AST/Type.cpp
@@ -3422,18 +3422,17 @@
 }
 
 DecltypeType::DecltypeType(Expr *E, QualType underlyingType, QualType can)
-    // C++11 [temp.type]p2: "If an expression e involves a template parameter,
-    // decltype(e) denotes a unique dependent type." Hence a decltype type is
-    // type-dependent even if its expression is only instantiation-dependent.
+    // C++17 [temp.type]p2: "If an expression e is type-dependent, decltype(e)
+    // denotes a unique dependent type."
     : Type(Decltype, can,
            toTypeDependence(E->getDependence()) |
-               (E->isInstantiationDependent() ? TypeDependence::Dependent
-                                              : TypeDependence::None) |
+               (E->isTypeDependent() ? TypeDependence::Dependent
+                                     : TypeDependence::None) |
                (E->getType()->getDependence() &
                 TypeDependence::VariablyModified)),
       E(E), UnderlyingType(underlyingType) {}
 
-bool DecltypeType::isSugared() const { return !E->isInstantiationDependent(); }
+bool DecltypeType::isSugared() const { return !E->isTypeDependent(); }
 
 QualType DecltypeType::desugar() const {
   if (isSugared())
diff --git a/clang/test/CodeGenCXX/mangle-exprs.cpp b/clang/test/CodeGenCXX/mangle-exprs.cpp
--- a/clang/test/CodeGenCXX/mangle-exprs.cpp
+++ b/clang/test/CodeGenCXX/mangle-exprs.cpp
@@ -215,7 +215,7 @@
 namespace test5 {
   template <typename T> void a(decltype(noexcept(T()))) {}
   template void a<int>(decltype(noexcept(int())));
-  // CHECK: void @_ZN5test51aIiEEvDTnxcvT__EE(
+  // CHECK: void @_ZN5test51aIiEEvb
 }
 
 namespace test6 {
diff --git a/clang/test/CodeGenCXX/mangle.cpp b/clang/test/CodeGenCXX/mangle.cpp
--- a/clang/test/CodeGenCXX/mangle.cpp
+++ b/clang/test/CodeGenCXX/mangle.cpp
@@ -802,7 +802,7 @@
   template<typename T>
   void f(decltype(sizeof(decltype(T() + T())))) {}
 
-  // CHECK-LABEL: define weak_odr void @_ZN6test341fIiEEvDTstDTplcvT__EcvS1__EEE
+  // CHECK-LABEL: define weak_odr void @_ZN6test341fIiEEvm
   template void f<int>(decltype(sizeof(1)));
 
   // Mangling for non-instantiation-dependent sizeof expressions.
@@ -839,7 +839,7 @@
   template<typename T>
   void f1(decltype(sizeof(&T::template operator+<int>))) {}
 
-  // CHECK-LABEL: define weak_odr void @_ZN6test352f1INS_1AEEEvDTszadsrT_onplIiEE
+  // CHECK-LABEL: define weak_odr void @_ZN6test352f1INS_1AEEEvm
   template void f1<A>(__SIZE_TYPE__);
 }
 
@@ -1116,14 +1116,14 @@
 namespace test56 {
   struct A { A *operator->(); int n; } a;
   template<int N> void f(decltype(a->n + N)) {}
-  // CHECK-LABEL: @_ZN6test561fILi0EEEvDTplptL_ZNS_1aEE1nT_E
+  // CHECK-LABEL: @_ZN6test561fILi0EEEvi
   template void f<0>(int);
 }
 
 namespace test57 {
   struct X { template <int N> int f(); } x;
   template<int N> void f(decltype(x.f<0>() + N)) {}
-  // CHECK-LABEL: @_ZN6test571fILi0EEEvDTplcldtL_ZNS_1xEE1fIXLi0EEEET_E
+  // CHECK-LABEL: @_ZN6test571fILi0EEEvi
   template void f<0>(int);
 }
 
diff --git a/clang/test/SemaCXX/invalid-template-base-specifier.cpp b/clang/test/SemaCXX/invalid-template-base-specifier.cpp
--- a/clang/test/SemaCXX/invalid-template-base-specifier.cpp
+++ b/clang/test/SemaCXX/invalid-template-base-specifier.cpp
@@ -12,11 +12,12 @@
 template <typename T>
 using Alias = decltype(Foo(T())); // expected-error {{no matching function for call to 'Foo'}}
 template <typename T>
-struct Crash2 : decltype(Alias<T>()) { // expected-note {{in instantiation of template type alias 'Alias' requested here}}
+struct Crash2 : decltype(Alias<T>()) { // expected-note {{in instantiation of template type alias 'Alias' requested here}} \
+                                          expected-error {{base specifier must name a class}}
   Crash2(){};
 };
 
-void test2() { Crash2<int>(); } // expected-note {{in instantiation of template class 'Crash2<int>' requested here}}
+void test2() { Crash2<int>(); } // expected-note2 {{in instantiation of template class 'Crash2<int>' requested here}}
 
 template <typename T>
 class Base {};
diff --git a/clang/test/SemaTemplate/dependent-expr.cpp b/clang/test/SemaTemplate/dependent-expr.cpp
--- a/clang/test/SemaTemplate/dependent-expr.cpp
+++ b/clang/test/SemaTemplate/dependent-expr.cpp
@@ -129,7 +129,7 @@
   template<typename> void f() {
     decltype(({})) x; // expected-error {{incomplete type}}
   }
-  template void f<int>(); // expected-note {{instantiation of}}
+  template void f<int>();
 
   template<typename> auto g() {
     auto c = [](auto, int) -> decltype(({})) {};