Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -3692,6 +3692,9 @@
       Expr *baseObjectExpr = nullptr,
       SourceLocation opLoc = SourceLocation());
 
+  void DiagnoseInstanceReference(const CXXScopeSpec &SS, NamedDecl *Rep,
+                                 const DeclarationNameInfo &nameInfo);
+
   ExprResult BuildPossibleImplicitMemberExpr(const CXXScopeSpec &SS,
                                              SourceLocation TemplateKWLoc,
                                              LookupResult &R,
Index: lib/Sema/SemaExprMember.cpp
===================================================================
--- lib/Sema/SemaExprMember.cpp
+++ lib/Sema/SemaExprMember.cpp
@@ -192,10 +192,8 @@
 }
 
 /// Diagnose a reference to a field with no object available.
-static void diagnoseInstanceReference(Sema &SemaRef,
-                                      const CXXScopeSpec &SS,
-                                      NamedDecl *Rep,
-                                      const DeclarationNameInfo &nameInfo) {
+void Sema::DiagnoseInstanceReference(const CXXScopeSpec &SS, NamedDecl *Rep,
+                                     const DeclarationNameInfo &nameInfo) {
   SourceLocation Loc = nameInfo.getLoc();
   SourceRange Range(Loc);
   if (SS.isSet()) Range.setBegin(SS.getRange().getBegin());
@@ -203,7 +201,7 @@
   // Look through using shadow decls and aliases.
   Rep = Rep->getUnderlyingDecl();
 
-  DeclContext *FunctionLevelDC = SemaRef.getFunctionLevelDeclContext();
+  DeclContext *FunctionLevelDC = getFunctionLevelDeclContext();
   CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(FunctionLevelDC);
   CXXRecordDecl *ContextClass = Method ? Method->getParent() : nullptr;
   CXXRecordDecl *RepClass = dyn_cast<CXXRecordDecl>(Rep->getDeclContext());
@@ -213,20 +211,19 @@
 
   if (IsField && InStaticMethod)
     // "invalid use of member 'x' in static member function"
-    SemaRef.Diag(Loc, diag::err_invalid_member_use_in_static_method)
+    Diag(Loc, diag::err_invalid_member_use_in_static_method)
         << Range << nameInfo.getName();
   else if (ContextClass && RepClass && SS.isEmpty() && !InStaticMethod &&
            !RepClass->Equals(ContextClass) && RepClass->Encloses(ContextClass))
     // Unqualified lookup in a non-static member function found a member of an
     // enclosing class.
-    SemaRef.Diag(Loc, diag::err_nested_non_static_member_use)
-      << IsField << RepClass << nameInfo.getName() << ContextClass << Range;
+    Diag(Loc, diag::err_nested_non_static_member_use)
+        << IsField << RepClass << nameInfo.getName() << ContextClass << Range;
   else if (IsField)
-    SemaRef.Diag(Loc, diag::err_invalid_non_static_member_use)
-      << nameInfo.getName() << Range;
+    Diag(Loc, diag::err_invalid_non_static_member_use) << nameInfo.getName()
+                                                       << Range;
   else
-    SemaRef.Diag(Loc, diag::err_member_call_without_object)
-      << Range;
+    Diag(Loc, diag::err_member_call_without_object) << Range;
 }
 
 /// Builds an expression which might be an implicit member expression.
@@ -260,7 +257,7 @@
 
   case IMA_Error_StaticContext:
   case IMA_Error_Unrelated:
-    diagnoseInstanceReference(*this, SS, R.getRepresentativeDecl(),
+    DiagnoseInstanceReference(SS, R.getRepresentativeDecl(),
                               R.getLookupNameInfo());
     return ExprError();
   }
@@ -474,7 +471,7 @@
   // If this is an implicit member access, use a different set of
   // diagnostics.
   if (!BaseExpr)
-    return diagnoseInstanceReference(SemaRef, SS, rep, nameInfo);
+    return SemaRef.DiagnoseInstanceReference(SS, rep, nameInfo);
 
   SemaRef.Diag(nameInfo.getLoc(), diag::err_qualified_member_of_unrelated)
     << SS.getRange() << rep << BaseType;
Index: lib/Sema/TreeTransform.h
===================================================================
--- lib/Sema/TreeTransform.h
+++ lib/Sema/TreeTransform.h
@@ -9128,8 +9128,18 @@
 
   // If we have neither explicit template arguments, nor the template keyword,
   // it's a normal declaration name.
-  if (!Old->hasExplicitTemplateArgs() && !TemplateKWLoc.isValid())
+  if (!Old->hasExplicitTemplateArgs() && !TemplateKWLoc.isValid()) {
+    // If an UnresolvedLookupExpr resolved to an instance member, that's an
+    // error.
+    NamedDecl *D = R.getAsSingle<NamedDecl>();
+    if (D && D->isCXXInstanceMember()) {
+      SemaRef.DiagnoseInstanceReference(SS, D, Old->getNameInfo());
+      R.clear();
+      return ExprError();
+    }
+
     return getDerived().RebuildDeclarationNameExpr(SS, R, Old->requiresADL());
+  }
 
   // If we have template arguments, rebuild them, then rebuild the
   // templateid expression.
Index: test/SemaCXX/using-decl-1.cpp
===================================================================
--- test/SemaCXX/using-decl-1.cpp
+++ test/SemaCXX/using-decl-1.cpp
@@ -327,3 +327,28 @@
     using PR24033::st; // expected-error {{target of using declaration conflicts with declaration already in scope}}
   }
 }
+
+namespace pr21923 {
+template <typename> struct Base {
+  int field;
+  void method();
+};
+template <typename Scalar> struct Derived : Base<Scalar> {
+  using Base<Scalar>::field;
+  using Base<Scalar>::method;
+  static void m_fn1() {
+    // expected-error@+1 {{invalid use of member 'field' in static member function}}
+    (void)field;
+    // expected-error@+1 {{invalid use of member 'field' in static member function}}
+    (void)&field;
+    // expected-error@+1 {{call to non-static member function without an object argument}}
+    (void)method;
+    // expected-error@+1 {{call to non-static member function without an object argument}}
+    (void)&method;
+    // expected-error@+1 {{call to non-static member function without an object argument}}
+    method();
+  }
+};
+// expected-note@+1 {{in instantiation of member function 'pr21923::Derived<int>::m_fn1' requested here}}
+template class Derived<int>;
+}
Index: test/SemaTemplate/instantiate-using-decl.cpp
===================================================================
--- test/SemaTemplate/instantiate-using-decl.cpp
+++ test/SemaTemplate/instantiate-using-decl.cpp
@@ -104,3 +104,26 @@
     x.f();
   }
 }
+
+namespace PR21923 {
+struct A {
+  int member;
+  void method();
+};
+template <typename T>
+struct B : public T {
+  using T::member;
+  using T::method;
+  static void StaticFun() {
+    (void)member; // expected-error {{invalid use of member 'member' in static member function}}
+    (void)B<T>::member; // expected-error {{invalid use of member 'member' in static member function}}
+    method(); // expected-error {{call to non-static member function without an object argument}}
+  }
+  void InstanceFun() {
+    (void)member;
+    (void)B<T>::member;
+    method();
+  }
+};
+template class B<A>; // expected-note 1 {{requested here}}
+}