4 files changed, 47 insertions, 9 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index c57a802512b..5897f2c4856 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -1269,11 +1269,36 @@ static bool CheckLValueConstantExpression(EvalInfo &Info, SourceLocation Loc,
           LVal.getLValueCallIndex() == 0) &&
          "have call index for global lvalue");
 
-  // Check if this is a thread-local variable.
   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>())
+        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>())
+        return false;
     }
   }
 
@@ -4373,11 +4398,8 @@ static bool EvaluateLValue(const Expr *E, LValue &Result, EvalInfo &Info) {
 }
 
 bool LValueExprEvaluator::VisitDeclRefExpr(const DeclRefExpr *E) {
-  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(E->getDecl())) {
-    if (FD->hasAttr<DLLImportAttr>())
-      return ZeroInitialization(E);
+  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(E->getDecl()))
     return Success(FD);
-  }
   if (const VarDecl *VD = dyn_cast<VarDecl>(E->getDecl()))
     return VisitVarDecl(E, VD);
   return Error(E);
@@ -4393,9 +4415,6 @@ bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
       Result.set(VD, Frame->Index);
       return true;
     }
-    // The address of __declspec(dllimport) variables aren't constant.
-    if (VD->hasAttr<DLLImportAttr>())
-      return ZeroInitialization(E);
     return Success(VD);
   }
 
diff --git a/clang/test/CodeGen/dllimport.c b/clang/test/CodeGen/dllimport.c
index 485f6e21b2a..32ee81f8592 100644
--- a/clang/test/CodeGen/dllimport.c
+++ b/clang/test/CodeGen/dllimport.c
@@ -53,7 +53,10 @@ USEVAR(GlobalRedecl3)
 // Import function declaration.
 // CHECK-DAG: declare dllimport void @decl()
 __declspec(dllimport) void decl(void);
-USE(decl)
+
+// Initialize use_decl with the address of the thunk.
+// CHECK-DAG: @use_decl = global void ()* @decl
+void (*use_decl)(void) = &decl;
 
 // Import inline function.
 // CHECK-DAG: declare dllimport void @inlineFunc()
diff --git a/clang/test/Sema/dllimport.c b/clang/test/Sema/dllimport.c
index f4d26fb8c87..2702453b61c 100644
--- a/clang/test/Sema/dllimport.c
+++ b/clang/test/Sema/dllimport.c
@@ -25,6 +25,9 @@ __declspec(dllimport) int GlobalDecl;
 int **__attribute__((dllimport))* GlobalDeclChunkAttr;
 int GlobalDeclAttr __attribute__((dllimport));
 
+// Address of variables can't be used for initialization in C language modes.
+int *VarForInit = &GlobalDecl; // expected-error{{initializer element is not a compile-time constant}}
+
 // Not allowed on definitions.
 __declspec(dllimport) extern int ExternGlobalInit = 1; // expected-error{{definition of dllimport data}}
 __declspec(dllimport) int GlobalInit1 = 1; // expected-error{{definition of dllimport data}}
@@ -96,6 +99,11 @@ __declspec(dllimport)      void decl1B();
 void __attribute__((dllimport)) decl2A();
 void __declspec(dllimport)      decl2B();
 
+// Address of functions can be used for initialization in C language modes.
+// However, the address of the thunk wrapping the function is used instead of
+// the address in the import address table.
+void (*FunForInit)() = &decl2A;
+
 // Not allowed on function definitions.
 __declspec(dllimport) void def() {} // expected-error{{dllimport cannot be applied to non-inline function definition}}
 
diff --git a/clang/test/SemaCXX/PR19955.cpp b/clang/test/SemaCXX/PR19955.cpp
index e0d4618f2c4..4596e5a459e 100644
--- a/clang/test/SemaCXX/PR19955.cpp
+++ b/clang/test/SemaCXX/PR19955.cpp
@@ -5,3 +5,11 @@ constexpr int *varp = &var; // expected-error {{must be initialized by a constan
 
 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;