Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -2228,12 +2228,16 @@
         // FIXME: Diagnostic!
         return false;
 
-      // A dllimport variable never acts like a constant, unless we're
-      // evaluating a value for use only in name mangling.
-      if (!isForManglingOnly(Kind) && Var->hasAttr<DLLImportAttr>())
+      if (!isForManglingOnly(Kind) && Var->hasAttr<DLLImportAttr>()) {
         // FIXME: Diagnostic!
-        return false;
-
+        if (Info.getLangOpts().C99)
+          // Address of dllimport variables can't be used for initialization in
+          // C language modes.
+          return false;
+        // A dllimport variable can be a constant address. This is a MS
+        // extension.
+        return true;
+      }
       // In CUDA/HIP device compilation, only device side variables have
       // constant addresses.
       if (Info.getCtx().getLangOpts().CUDA &&
Index: clang/test/CodeGenCXX/PR19955.cpp
===================================================================
--- clang/test/CodeGenCXX/PR19955.cpp
+++ clang/test/CodeGenCXX/PR19955.cpp
@@ -6,20 +6,15 @@
 
 extern int *varp;
 int *varp = &var;
-// CHECK-DAG: @"?varp@@3PAHA" = dso_local global ptr null
-// X64-DAG: @"?varp@@3PEAHEA" = dso_local global ptr null
+// CHECK-DAG: @"?var@@3HA" = external dllimport global i32
+// CHECK-DAG: @"?varp@@3PAHA" = dso_local global ptr @"?var@@3HA"
+
 
 extern void (*funp)();
 void (*funp)() = &fun;
 // CHECK-DAG: @"?funp@@3P6AXXZA" = dso_local global ptr null
 // X64-DAG: @"?funp@@3P6AXXZEA" = dso_local global ptr null
 
-// CHECK-LABEL: @"??__Evarp@@YAXXZ"
-// CHECK-DAG: store ptr @"?var@@3HA", ptr @"?varp@@3PAHA"
-
-// X64-LABEL: @"??__Evarp@@YAXXZ"
-// X64-DAG: store ptr @"?var@@3HA", ptr @"?varp@@3PEAHEA"
-
 // CHECK-LABEL: @"??__Efunp@@YAXXZ"()
 // CHECK-DAG: store ptr @"?fun@@YAXXZ", ptr @"?funp@@3P6AXXZA"
 
Index: clang/test/CodeGenCXX/dllimport.cpp
===================================================================
--- clang/test/CodeGenCXX/dllimport.cpp
+++ clang/test/CodeGenCXX/dllimport.cpp
@@ -97,9 +97,9 @@
 };
 USE(inlineStaticLocalsFunc);
 
-// The address of a dllimport global cannot be used in constant initialization.
-// M32-DAG: @"?arr@?1??initializationFunc@@YAPAHXZ@4QBQAHB" = internal global [1 x ptr] zeroinitializer
-// GNU-DAG: @_ZZ18initializationFuncvE3arr = internal global [1 x ptr] zeroinitializer
+// The address of a dllimport global can be used in constant initialization.
+// M32-DAG: @"?arr@?1??initializationFunc@@YAPAHXZ@4QBQAHB" = internal constant [1 x ptr] [ptr @"?ExternGlobalDecl@@3HA"]
+// GNU-DAG: @_ZZ18initializationFuncvE3arr = internal constant [1 x ptr] [ptr @ExternGlobalDecl]
 int *initializationFunc() {
   static int *const arr[] = {&ExternGlobalDecl};
   return arr[0];
Index: clang/test/SemaCXX/PR19955.cpp
===================================================================
--- clang/test/SemaCXX/PR19955.cpp
+++ clang/test/SemaCXX/PR19955.cpp
@@ -2,7 +2,7 @@
 // RUN: %clang_cc1 -triple i686-mingw32 -verify -std=c++11 %s
 
 extern int __attribute__((dllimport)) var;
-constexpr int *varp = &var; // expected-error {{must be initialized by a constant expression}}
+constexpr int *varp = &var;
 
 extern __attribute__((dllimport)) void fun();
 constexpr void (*funp)(void) = &fun; // expected-error {{must be initialized by a constant expression}}
Index: clang/test/SemaCXX/dllimport-constexpr.cpp
===================================================================
--- clang/test/SemaCXX/dllimport-constexpr.cpp
+++ clang/test/SemaCXX/dllimport-constexpr.cpp
@@ -40,7 +40,6 @@
 // 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;