[PATCH] D22053: [Sema] Fix a C++1z bug where initializer for static constexpr data member was not instantiated

Richard Smith via cfe-commits cfe-commits at lists.llvm.org
Wed Jul 13 12:27:35 PDT 2016

rsmith added inline comments.

Comment at: lib/Sema/SemaTemplateInstantiateDecl.cpp:3861
@@ -3860,3 +3860,3 @@
   // static data members until a definition of the variable is needed. We need
-  // it right away if the type contains 'auto'.
+  // it right away if the type contains 'auto' or is an IncompleteArrayType.
   if ((!isa<VarTemplateSpecializationDecl>(NewVar) &&
I don't see why we would need the initializer right away in the `IncompleteArrayType` case. It seems instead that we should delay instantiation of the initializer until a complete type is required for the variable (`Sema::RequireCompleteExprType` handles this case) or it is used in a context that requires a definition (`Sema::MarkVariableReferenced` handles this case).

You can reproduce a related bug in C++11 mode like so:

    template<typename T> struct X { static const int arr[]; };
    template<typename T> constexpr int X<T>::arr[] = {1, 2, 3};
    constexpr int k = X<int>::arr[0];


    template<typename T> struct X { static const double n; };
    template<typename T> constexpr double X<T>::n = 1;
    template<int> struct Y {};
    Y<(int)X<int>::n> y;

It looks like the bug in this case is that `VarDecl::isUsableInConstantExpressions` (called from `DoMarkVarDeclReferenced`) is mishandling this case: once it's determined that the variable is of non-`volatile` `const`-qualified type, it needs to map back to the template instantiation pattern and check whether the most recent declaration of that is declared `constexpr`, since there might be a not-yet-instantiated redeclaration that adds the `constexpr`.

I'm not sure that's the same bug you're hitting here, though, since in this case we should be instantiating the `constexpr` specifier with the initial declaration, which should be enough to cause `isUsableInConstantExpressions` to return `true`. So the mystery is, why is `DoMarkVarDeclReferenced` not triggering instantiation?


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