Don't allow dllimport variables in constant initializers

[hans]

Summary

This is a follow-up to David's r211677. For the following code, we would end up referring to 'foo' in the initializer for 'arr', and then fail to link, because 'foo' is dllimport and needs to be accessed through the __imp_?foo.

[majnemer]

LGTM.

[hans]

Closed by commit rL211736 (authored by @hans).

[rnk]

This seems problematic for mingw, where I expect users probably use non-type template parameters of dllimported stuff. Is there another way we can handle this?

lib/AST/ExprConstant.cpp

assert((Info.checkingPotentialConstantExpression() ||
          "have call index for global lvalue");
 
   if (const ValueDecl *VD = Base.dyn_cast<const ValueDecl*>()) {
     if (const VarDecl *Var = dyn_cast<const VarDecl>(VD)) {
       // Check if this is a thread-local variable.
       if (Var->getTLSKind())
         return false;
 
-      // Check if this is a dllimport variable.  Fail evaluation if we care
-      // about side effects; a dllimport variable rarely acts like a constant
-      // except in places like template arguments.  It never acts like a
-      // constant in C.
-      if ((!Info.getLangOpts().CPlusPlus ||
-           !Info.keepEvaluatingAfterSideEffect()) &&
-          Var->hasAttr<DLLImportAttr>())
+      // A dllimport variable never acts like a constant.
+      if (Var->hasAttr<DLLImportAttr>())
         return false;
     }
     if (const auto *FD = dyn_cast<const FunctionDecl>(VD)) {
       // __declspec(dllimport) must be handled very carefully:
       // We must never initialize an expression with the thunk in C++.
       // Doing otherwise would allow the same id-expression to yield
       // different addresses for the same function in different translation
       // units.  However, this means that we must dynamically initialize the
       // expression with the contents of the import address table at runtime.
       //
       // 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 &&
-          !Info.keepEvaluatingAfterSideEffect() &&
-          FD->hasAttr<DLLImportAttr>())
+      if (Info.getLangOpts().CPlusPlus && FD->hasAttr<DLLImportAttr>())
         return false;
     }
   }
 
   // Allow address constant expressions to be past-the-end pointers. This is
   // an extension: the standard requires them to point to an object.
   if (!IsReferenceType)
     return true;

lib/Sema/SemaTemplate.cpp

 /// \brief Determine whether the given template argument is a null pointer
 /// value of the appropriate type.
 static NullPointerValueKind
 isNullPointerValueTemplateArgument(Sema &S, NonTypeTemplateParmDecl *Param,
                                    QualType ParamType, Expr *Arg) {
   if (Arg->isValueDependent() || Arg->isTypeDependent())
     return NPV_NotNullPointer;
   
-  if (!S.getLangOpts().CPlusPlus11)
+  if (!S.getLangOpts().CPlusPlus11 || S.getLangOpts().MSVCCompat)
     return NPV_NotNullPointer;
   
   // Determine whether we have a constant expression.
   ExprResult ArgRV = S.DefaultFunctionArrayConversion(Arg);
   if (ArgRV.isInvalid())
     return NPV_Error;
   Arg = ArgRV.get();
   

test/CodeGenCXX/dllimport.cpp

 // MO1-DAG: @"\01?x@?1??inlineStaticLocalsFunc@@YAHXZ@4HA" = available_externally dllimport global i32 0
 // MO1-DAG: @"\01??_B?1??inlineStaticLocalsFunc@@YAHXZ@51" = available_externally dllimport global i32 0
 inline int __declspec(dllimport) inlineStaticLocalsFunc() {
   static int x = f();
   return x++;
 };
 USE(inlineStaticLocalsFunc);
 
+// The address of a dllimport global cannot be used in constant initialization.
+// M32-DAG: @"\01?arr@?0??initializationFunc@@YAPAHXZ@4QBQAHB" = internal global [1 x i32*] zeroinitializer
+// GNU-DAG: @_ZZ18initializationFuncvE3arr = internal global [1 x i32*] zeroinitializer
+int *initializationFunc() {
+  static int *const arr[] = {&ExternGlobalDecl};
+  return arr[0];
+}
+USE(initializationFunc);
 
 
 //===----------------------------------------------------------------------===//
 // Variable templates
 //===----------------------------------------------------------------------===//
 
 // Import declaration.
 // MSC-DAG: @"\01??$ExternVarTmplDecl@UImplicitInst_Imported@@@@3HA" = external dllimport global i32

test/Parser/MicrosoftExtensions.cpp

typedef COM_CLASS_TEMPLATE_REF<struct_with_uuid, __uuidof(struct_with_uuid)> COM_TYPE_REF;
   template<typename T>
   void test_late_defined_uuid() {
     __uuidof(late_defined_uuid);
   }
   struct __declspec(uuid("000000A0-0000-0000-C000-000000000049")) late_defined_uuid;
 
 COM_CLASS_TEMPLATE_REF<int, __uuidof(struct_with_uuid)> good_template_arg;
 
-COM_CLASS_TEMPLATE<int, __uuidof(struct_with_uuid)> bad_template_arg; // expected-error {{non-type template argument of type 'const _GUID' is not a constant expression}}
+COM_CLASS_TEMPLATE<int, __uuidof(struct_with_uuid)> bad_template_arg; // expected-error {{non-type template argument of type 'const _GUID' cannot be converted to a value of type 'const GUID *' (aka 'const _GUID *')}}
 
 namespace PR16911 {
 struct __declspec(uuid("{12345678-1234-1234-1234-1234567890aB}")) uuid;
 struct __declspec(uuid("{12345678-1234-1234-1234-1234567890aB}")) uuid2;
 
 template <typename T, typename T2>
 struct thing {
 };

test/SemaCXX/PR19955.cpp

-// RUN: %clang_cc1 -triple i686-win32 -verify -std=c++11 %s
+// RUN: %clang_cc1 -triple i686-win32 -fms-compatibility -verify -std=c++11 %s
 
 extern int __attribute__((dllimport)) var;
 constexpr int *varp = &var; // expected-error {{must be initialized by a constant expression}}
 
 extern __attribute__((dllimport)) void fun();
 constexpr void (*funp)(void) = &fun; // expected-error {{must be initialized by a constant expression}}
 
 template <void (*)()>
 struct S {};
 S<&fun> x;
 
 template <int *>
 struct U {};
 U<&var> y;