AST: Address of dllimport variables isn't constant

[majnemer]

Summary

The address of dllimport variables isn't something that can be
meaningfully used in a constexpr context and isn't suitable for
evaluation at load-time. They require loads from memory to properly
evaluate.

This fixes PR19955.

[rnk]

Can you add a sema test for 'extern int __declspec(dllimport) y; constexpr int *x = &y;'?

lib/AST/ExprConstant.cpp

4395

How is ZeroInitialization(E) different from Error(E)?

[majnemer]

Sure, I'll add a SemaCXX test for that.

lib/AST/ExprConstant.cpp

4395

It isn't other than being, in my opinion, less confusing. This isn't actually an error per-se because it isn't outright nonsense, it's just not evaluatable. This matches what the codebase does for:

int x = foo();

where x is at file scope and foo isn't constexpr.

[majnemer]

• Add a SemaCXX test for constexpr + dllimport

[rnk]

lgtm

[majnemer]

Closed by commit rL211568 (authored by @majnemer).

lib/AST/ExprConstant.cpp

bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
   if (VD->hasLocalStorage() && Info.CurrentCall->Index > 1)
     Frame = Info.CurrentCall;
 
   if (!VD->getType()->isReferenceType()) {
     if (Frame) {
       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);
   }
 
   APValue *V;
   if (!evaluateVarDeclInit(Info, E, VD, Frame, V))
     return false;
   if (V->isUninit()) {
     if (!Info.checkingPotentialConstantExpression())

test/CodeGenCXX/PR19955.cpp

+// RUN: %clang_cc1 -triple i686-windows-msvc -fno-rtti -emit-llvm -std=c++1y -O0 -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -fno-rtti -emit-llvm -std=c++1y -O0 -o - %s | FileCheck %s
+
+extern int __declspec(dllimport) x;
+extern long long y;
+// CHECK-DAG: @"\01?y@@3_JA" = global i64 0
+long long y = (long long)&x;
+
+// CHECK-LABEL: @"\01??__Ey@@YAXXZ"
+// CHECK-DAG: @"\01?y@@3_JA"

test/SemaCXX/PR19955.cpp

+// RUN: %clang_cc1 -triple i686-win32 -verify -std=c++11 %s
+
+extern int __attribute__((dllimport)) y;
+constexpr int *x = &y; // expected-error {{must be initialized by a constant expression}}