Index: lib/Sema/SemaTemplate.cpp
===================================================================
--- lib/Sema/SemaTemplate.cpp
+++ lib/Sema/SemaTemplate.cpp
@@ -7367,6 +7367,26 @@
     }
   }
 
+  // In MSVC mode, dllimported explicit instantiation definitions are treated as
+  // instantiation declarations for most purposes.
+  bool DLLImportExplicitInstantiationDef = false;
+  if (TSK == TSK_ExplicitInstantiationDefinition &&
+      Context.getTargetInfo().getCXXABI().isMicrosoft()) {
+    // Check for dllimport class template instantiation definitions.
+    bool DLLImport =
+        ClassTemplate->getTemplatedDecl()->getAttr<DLLImportAttr>();
+    for (AttributeList *A = Attr; A; A = A->getNext()) {
+      if (A->getKind() == AttributeList::AT_DLLImport)
+        DLLImport = true;
+      if (A->getKind() == AttributeList::AT_DLLExport)
+        DLLImport = false;
+    }
+    if (DLLImport) {
+      TSK = TSK_ExplicitInstantiationDeclaration;
+      DLLImportExplicitInstantiationDef = true;
+    }
+  }
+
   // Translate the parser's template argument list in our AST format.
   TemplateArgumentListInfo TemplateArgs(LAngleLoc, RAngleLoc);
   translateTemplateArguments(TemplateArgsIn, TemplateArgs);
@@ -7441,6 +7461,12 @@
     }
   }
 
+  if (PrevDecl_TSK == TSK_ExplicitInstantiationDeclaration &&
+      DLLImportExplicitInstantiationDef) {
+    // The new specialization might add a dllimport attribute.
+    HasNoEffect = false;
+  }
+
   // Build the fully-sugared type for this explicit instantiation as
   // the user wrote in the explicit instantiation itself. This means
   // that we'll pretty-print the type retrieved from the
@@ -7497,11 +7523,11 @@
                                        Specialization->getDefinition());
   if (Def) {
     TemplateSpecializationKind Old_TSK = Def->getTemplateSpecializationKind();
-
     // Fix a TSK_ExplicitInstantiationDeclaration followed by a
     // TSK_ExplicitInstantiationDefinition
     if (Old_TSK == TSK_ExplicitInstantiationDeclaration &&
-        TSK == TSK_ExplicitInstantiationDefinition) {
+        (TSK == TSK_ExplicitInstantiationDefinition ||
+         DLLImportExplicitInstantiationDef)) {
       // FIXME: Need to notify the ASTMutationListener that we did this.
       Def->setTemplateSpecializationKind(TSK);
 
Index: test/CodeGenCXX/dllexport.cpp
===================================================================
--- test/CodeGenCXX/dllexport.cpp
+++ test/CodeGenCXX/dllexport.cpp
@@ -958,14 +958,6 @@
 // M32-DAG: define weak_odr dllexport x86_thiscallcc void @"\01?func@?$ExplicitInstantiationDeclTemplateBase@H@@QAEXXZ"
 // G32-DAG: define weak_odr x86_thiscallcc void @_ZN37ExplicitInstantiationDeclTemplateBaseIiE4funcEv
 
-template <typename T> struct ExplicitInstantiationDeclTemplateBase2 { void func() {} };
-extern template struct ExplicitInstantiationDeclTemplateBase2<int>;
-struct __declspec(dllexport) DerivedFromExplicitInstantiationDeclTemplateBase2 : public ExplicitInstantiationDeclTemplateBase2<int> {};
-template struct __declspec(dllimport) ExplicitInstantiationDeclTemplateBase2<int>;
-USEMEMFUNC(ExplicitInstantiationDeclTemplateBase2<int>, func)
-// M32-DAG: define weak_odr dllexport x86_thiscallcc void @"\01?func@?$ExplicitInstantiationDeclTemplateBase2@H@@QAEXXZ"
-// G32-DAG: define weak_odr x86_thiscallcc void @_ZN38ExplicitInstantiationDeclTemplateBase2IiE4funcEv
-
 // PR26076
 struct LayerSelectionBound;
 template <typename> struct Selection {};
Index: test/CodeGenCXX/dllimport.cpp
===================================================================
--- test/CodeGenCXX/dllimport.cpp
+++ test/CodeGenCXX/dllimport.cpp
@@ -656,7 +656,7 @@
 
 namespace Vtordisp {
   // Don't dllimport the vtordisp.
-  // MO1-DAG: define linkonce_odr x86_thiscallcc void @"\01?f@?$C@D@Vtordisp@@$4PPPPPPPM@A@AEXXZ"
+  // MO1-DAG: define linkonce_odr x86_thiscallcc void @"\01?f@?$C@H@Vtordisp@@$4PPPPPPPM@A@AEXXZ"
 
   class Base {
     virtual void f() {}
@@ -667,7 +667,7 @@
     C() {}
     virtual void f() {}
   };
-  template class C<char>;
+  USECLASS(C<int>);
 }
 
 namespace ClassTemplateStaticDef {
@@ -698,26 +698,31 @@
   template <typename T> struct A { static NonPOD x; };
   template <typename T> NonPOD A<T>::x;
   template struct __declspec(dllimport) A<int>;
-  // MSC-DAG: @"\01?x@?$A@H@PR19933@@2UNonPOD@2@A" = available_externally dllimport global %"struct.PR19933::NonPOD" zeroinitializer
+  USEVARTYPE(NonPOD, A<int>::x);
+  // MSC-DAG: @"\01?x@?$A@H@PR19933@@2UNonPOD@2@A" = external dllimport global %"struct.PR19933::NonPOD"
 
   int f();
   template <typename T> struct B { static int x; };
   template <typename T> int B<T>::x = f();
   template struct __declspec(dllimport) B<int>;
-  // MSC-DAG: @"\01?x@?$B@H@PR19933@@2HA" = available_externally dllimport global i32 0
+  USEVAR(B<int>::x);
+  // MSC-DAG: @"\01?x@?$B@H@PR19933@@2HA" = external dllimport global i32
 
   constexpr int g() { return 42; }
   template <typename T> struct C { static int x; };
   template <typename T> int C<T>::x = g();
   template struct __declspec(dllimport) C<int>;
-  // MSC-DAG: @"\01?x@?$C@H@PR19933@@2HA" = available_externally dllimport global i32 42
+  USEVAR(C<int>::x);
+  // MSC-DAG: @"\01?x@?$C@H@PR19933@@2HA" = external dllimport global i32
 
   template <int I> struct D { static int x, y; };
   template <int I> int D<I>::x = I + 1;
   template <int I> int D<I>::y = I + f();
   template struct __declspec(dllimport) D<42>;
-  // MSC-DAG: @"\01?x@?$D@$0CK@@PR19933@@2HA" = available_externally dllimport global i32 43
-  // MSC-DAG: @"\01?y@?$D@$0CK@@PR19933@@2HA" = available_externally dllimport global i32 0
+  USEVAR(D<42>::x);
+  USEVAR(D<42>::y);
+  // MSC-DAG: @"\01?x@?$D@$0CK@@PR19933@@2HA" = external dllimport global i32
+  // MSC-DAG: @"\01?y@?$D@$0CK@@PR19933@@2HA" = external dllimport global i32
 }
 
 namespace PR21355 {
@@ -805,6 +810,36 @@
 USEMEMFUNC(PR23770BaseTemplate<int>, f);
 // M32-DAG: declare dllimport x86_thiscallcc void @"\01?f@?$PR23770BaseTemplate@H@@QAEXXZ"
 
+namespace PR27810 {
+  template <class T>
+  struct basic_ostream {
+    struct sentry {
+      sentry() { }
+      void foo() { }
+    };
+  };
+  template class __declspec(dllimport) basic_ostream<char>;
+  // The explicit instantiation definition acts as an explicit instantiation
+  // *declaration*, dllimport is not inherited by the inner class, and no
+  // functions are emitted unless they are used.
+
+  USEMEMFUNC(basic_ostream<char>::sentry, foo);
+  // M32-DAG: {{declare|define available_externally}} x86_thiscallcc void @"\01?foo@sentry@?$basic_ostream@D@PR27810@@QAEXXZ"
+  // M32-NOT: ??0sentry@?$basic_ostream@D@PR27810@@QAE@XZ
+}
+
+namespace PR27811 {
+  template <class T> struct codecvt {
+    virtual ~codecvt() { }
+  };
+  template class __declspec(dllimport) codecvt<char>;
+
+  // dllimport means this explicit instantiation definition gets treated as a
+  // declaration. Thus, the vtable should not be marked used, and in fact
+  // nothing for this class should be emitted at all since it's not used.
+  // M32-NOT: codecvt
+}
+
 //===----------------------------------------------------------------------===//
 // Classes with template base classes
 //===----------------------------------------------------------------------===//