Index: include/clang/AST/Expr.h
===================================================================
--- include/clang/AST/Expr.h
+++ include/clang/AST/Expr.h
@@ -658,6 +658,13 @@
                                 ArrayRef<const Expr*> Args,
                                 const Expr *This = nullptr) const;
 
+  /// Indicates how the constant expression will be used.
+  enum ConstExprUsage { EvaluateForCodeGen, EvaluateForMangling };
+
+  /// Evaluate an expression that is required to be a constant expression.
+  bool EvaluateAsConstantExpr(EvalResult &Result, ConstExprUsage Usage,
+                              const ASTContext &Ctx) const;
+
   /// If the current Expr is a pointer, this will try to statically
   /// determine the number of bytes available where the pointer is pointing.
   /// Returns true if all of the above holds and we were able to figure out the
Index: lib/AST/ExprConstant.cpp
===================================================================
--- lib/AST/ExprConstant.cpp
+++ lib/AST/ExprConstant.cpp
@@ -1720,7 +1720,8 @@
 /// value for an address or reference constant expression. Return true if we
 /// can fold this expression, whether or not it's a constant expression.
 static bool CheckLValueConstantExpression(EvalInfo &Info, SourceLocation Loc,
-                                          QualType Type, const LValue &LVal) {
+                                          QualType Type, const LValue &LVal,
+                                          Expr::ConstExprUsage Usage) {
   bool IsReferenceType = Type->isReferenceType();
 
   APValue::LValueBase Base = LVal.getLValueBase();
@@ -1753,7 +1754,7 @@
         return false;
 
       // A dllimport variable never acts like a constant.
-      if (Var->hasAttr<DLLImportAttr>())
+      if (Usage == Expr::EvaluateForCodeGen && Var->hasAttr<DLLImportAttr>())
         return false;
     }
     if (const auto *FD = dyn_cast<const FunctionDecl>(VD)) {
@@ -1767,7 +1768,8 @@
       // The C language has no notion of ODR; furthermore, it has no notion of
       // dynamic initialization.  This means that we are permitted to
       // perform initialization with the address of the thunk.
-      if (Info.getLangOpts().CPlusPlus && FD->hasAttr<DLLImportAttr>())
+      if (Info.getLangOpts().CPlusPlus && Usage == Expr::EvaluateForCodeGen &&
+          FD->hasAttr<DLLImportAttr>())
         return false;
     }
   }
@@ -1800,12 +1802,14 @@
 static bool CheckMemberPointerConstantExpression(EvalInfo &Info,
                                                  SourceLocation Loc,
                                                  QualType Type,
-                                                 const APValue &Value) {
+                                                 const APValue &Value,
+                                                 Expr::ConstExprUsage Usage) {
   const ValueDecl *Member = Value.getMemberPointerDecl();
   const auto *FD = dyn_cast_or_null<CXXMethodDecl>(Member);
   if (!FD)
     return true;
-  return FD->isVirtual() || !FD->hasAttr<DLLImportAttr>();
+  return Usage == Expr::EvaluateForMangling || FD->isVirtual() ||
+         !FD->hasAttr<DLLImportAttr>();
 }
 
 /// Check that this core constant expression is of literal type, and if not,
@@ -1843,8 +1847,10 @@
 /// Check that this core constant expression value is a valid value for a
 /// constant expression. If not, report an appropriate diagnostic. Does not
 /// check that the expression is of literal type.
-static bool CheckConstantExpression(EvalInfo &Info, SourceLocation DiagLoc,
-                                    QualType Type, const APValue &Value) {
+static bool
+CheckConstantExpression(EvalInfo &Info, SourceLocation DiagLoc, QualType Type,
+                        const APValue &Value,
+                        Expr::ConstExprUsage Usage = Expr::EvaluateForCodeGen) {
   if (Value.isUninit()) {
     Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized)
       << true << Type;
@@ -1863,28 +1869,28 @@
     QualType EltTy = Type->castAsArrayTypeUnsafe()->getElementType();
     for (unsigned I = 0, N = Value.getArrayInitializedElts(); I != N; ++I) {
       if (!CheckConstantExpression(Info, DiagLoc, EltTy,
-                                   Value.getArrayInitializedElt(I)))
+                                   Value.getArrayInitializedElt(I), Usage))
         return false;
     }
     if (!Value.hasArrayFiller())
       return true;
-    return CheckConstantExpression(Info, DiagLoc, EltTy,
-                                   Value.getArrayFiller());
+    return CheckConstantExpression(Info, DiagLoc, EltTy, Value.getArrayFiller(),
+                                   Usage);
   }
   if (Value.isUnion() && Value.getUnionField()) {
     return CheckConstantExpression(Info, DiagLoc,
                                    Value.getUnionField()->getType(),
-                                   Value.getUnionValue());
+                                   Value.getUnionValue(), Usage);
   }
   if (Value.isStruct()) {
     RecordDecl *RD = Type->castAs<RecordType>()->getDecl();
     if (const CXXRecordDecl *CD = dyn_cast<CXXRecordDecl>(RD)) {
       unsigned BaseIndex = 0;
-      for (CXXRecordDecl::base_class_const_iterator I = CD->bases_begin(),
-             End = CD->bases_end(); I != End; ++I, ++BaseIndex) {
-        if (!CheckConstantExpression(Info, DiagLoc, I->getType(),
-                                     Value.getStructBase(BaseIndex)))
+      for (const CXXBaseSpecifier &BS : CD->bases()) {
+        if (!CheckConstantExpression(Info, DiagLoc, BS.getType(),
+                                     Value.getStructBase(BaseIndex), Usage))
           return false;
+        ++BaseIndex;
       }
     }
     for (const auto *I : RD->fields()) {
@@ -1892,7 +1898,8 @@
         continue;
 
       if (!CheckConstantExpression(Info, DiagLoc, I->getType(),
-                                   Value.getStructField(I->getFieldIndex())))
+                                   Value.getStructField(I->getFieldIndex()),
+                                   Usage))
         return false;
     }
   }
@@ -1900,11 +1907,11 @@
   if (Value.isLValue()) {
     LValue LVal;
     LVal.setFrom(Info.Ctx, Value);
-    return CheckLValueConstantExpression(Info, DiagLoc, Type, LVal);
+    return CheckLValueConstantExpression(Info, DiagLoc, Type, LVal, Usage);
   }
 
   if (Value.isMemberPointer())
-    return CheckMemberPointerConstantExpression(Info, DiagLoc, Type, Value);
+    return CheckMemberPointerConstantExpression(Info, DiagLoc, Type, Value, Usage);
 
   // Everything else is fine.
   return true;
@@ -10345,13 +10352,25 @@
   LValue LV;
   if (!EvaluateLValue(this, LV, Info) || Result.HasSideEffects ||
       !CheckLValueConstantExpression(Info, getExprLoc(),
-                                     Ctx.getLValueReferenceType(getType()), LV))
+                                     Ctx.getLValueReferenceType(getType()), LV,
+                                     Expr::EvaluateForCodeGen))
     return false;
 
   LV.moveInto(Result.Val);
   return true;
 }
 
+bool Expr::EvaluateAsConstantExpr(EvalResult &Result, ConstExprUsage Usage,
+                                  const ASTContext &Ctx) const {
+  EvalInfo::EvaluationMode EM = EvalInfo::EM_ConstantExpression;
+  EvalInfo Info(Ctx, Result, EM);
+  if (!::Evaluate(Result.Val, Info, this))
+    return false;
+
+  return CheckConstantExpression(Info, getExprLoc(), getType(), Result.Val,
+                                 Usage);
+}
+
 bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
                                  const VarDecl *VD,
                             SmallVectorImpl<PartialDiagnosticAt> &Notes) const {
Index: lib/Sema/SemaOverload.cpp
===================================================================
--- lib/Sema/SemaOverload.cpp
+++ lib/Sema/SemaOverload.cpp
@@ -5407,10 +5407,11 @@
   SmallVector<PartialDiagnosticAt, 8> Notes;
   Expr::EvalResult Eval;
   Eval.Diag = &Notes;
+  Expr::ConstExprUsage Usage = CCE == Sema::CCEK_TemplateArg
+                                   ? Expr::EvaluateForMangling
+                                   : Expr::EvaluateForCodeGen;
 
-  if ((T->isReferenceType()
-           ? !Result.get()->EvaluateAsLValue(Eval, S.Context)
-           : !Result.get()->EvaluateAsRValue(Eval, S.Context)) ||
+  if (!Result.get()->EvaluateAsConstantExpr(Eval, Usage, S.Context) ||
       (RequireInt && !Eval.Val.isInt())) {
     // The expression can't be folded, so we can't keep it at this position in
     // the AST.
Index: test/SemaCXX/dllimport-constexpr.cpp
===================================================================
--- test/SemaCXX/dllimport-constexpr.cpp
+++ test/SemaCXX/dllimport-constexpr.cpp
@@ -0,0 +1,62 @@
+// RUN: %clang_cc1 -std=c++14 %s -verify -fms-extensions -triple x86_64-windows-msvc
+// RUN: %clang_cc1 -std=c++17 %s -verify -fms-extensions -triple x86_64-windows-msvc
+
+__declspec(dllimport) void imported_func();
+__declspec(dllimport) int imported_int;
+struct Foo {
+  void __declspec(dllimport) imported_method();
+};
+
+// Instantiation is OK.
+template <void (*FP)()> struct TemplateFnPtr {
+  static void getit() { FP(); }
+};
+template <void (&FP)()> struct TemplateFnRef {
+  static void getit() { FP(); }
+};
+void instantiate1() {
+  TemplateFnPtr<&imported_func>::getit();
+  TemplateFnRef<imported_func>::getit();
+}
+
+// Check variable template instantiation.
+template <int *GI> struct TemplateIntPtr {
+  static int getit() { return *GI; }
+};
+template <int &GI> struct TemplateIntRef {
+  static int getit() { return GI; }
+};
+int instantiate2() {
+  int r = 0;
+  r += TemplateIntPtr<&imported_int>::getit();
+  r += TemplateIntRef<imported_int>::getit();
+  return r;
+}
+
+// Member pointer instantiation.
+template <void (Foo::*MP)()> struct TemplateMemPtr { };
+TemplateMemPtr<&Foo::imported_method> instantiate_mp;
+
+// constexpr initialization doesn't work for dllimport things.
+// expected-error@+1{{must be initialized by a constant expression}}
+constexpr void (*constexpr_import_func)() = &imported_func;
+// expected-error@+1{{must be initialized by a constant expression}}
+constexpr int *constexpr_import_int = &imported_int;
+// expected-error@+1{{must be initialized by a constant expression}}
+constexpr void (Foo::*constexpr_memptr)() = &Foo::imported_method;
+
+// We make dynamic initializers for 'const' globals, but not constexpr ones.
+void (*const const_import_func)() = &imported_func;
+int *const const_import_int = &imported_int;
+void (Foo::*const const_memptr)() = &Foo::imported_method;
+
+// Check that using a non-type template parameter for constexpr global
+// initialization is correctly diagnosed during template instantiation.
+template <void (*FP)()> struct StaticConstexpr {
+  // expected-error@+1{{must be initialized by a constant expression}}
+  static constexpr void (*g_fp)() = FP;
+};
+void instantiate3() {
+  // expected-note@+1 {{requested here}}
+  StaticConstexpr<imported_func>::g_fp();
+}
Index: test/SemaCXX/dllimport-memptr.cpp
===================================================================
--- test/SemaCXX/dllimport-memptr.cpp
+++ test/SemaCXX/dllimport-memptr.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -triple x86_64-windows-msvc -fms-extensions -verify -std=c++11 %s
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -fms-extensions -verify -std=c++17 %s
 
 // expected-no-diagnostics