Name conflict between namespace and class template: different compiler behavior

Question

Different compilers show different behavior compiling the following code:

namespace N
{
    namespace Foo
    {
        template <typename>
        struct Foo
        {
        };
    }
}

template <typename Type>
using Foo = N::Foo::Foo<Type>;

namespace N
{
    template <typename Type>
    struct Bar : Foo<Type>
    {
    };
}


int main()
{
}

Compilers tested and their compilation flags:

• clang++ 5.0.0: -std=c++14 -Wall -Wextra -Werror -pedantic-errors
• g++ 7.2: -std=c++14 -Wall -Wextra -Werror -pedantic-errors
• vc++ 19.10.25017 (VS 2017): /EHsc /Za /std:c++14 /permissive-
• icc 18.0.0: -std=c++14 -Wall -Werror

Results of the compilation:

• clang++:

  18 : <source>:18:15: error: expected class name
          struct Bar : Foo
                       ^
  

• g++: successful compilation

• vc++:

  18 : <source>(18): error C2516: 'Foo': is not a legal base class
  13 : <source>(13): note: see declaration of 'Foo'
  20 : <source>(20): note: see reference to class template instantiation 'N::Bar<Type>' being compiled
  18 : <source>(18): error C2143: syntax error: missing ',' before '<'
  

• icc: successful compilation

Which compiler behavior is standard compliant?

Additional information:

• same code with anonymous namespaces
• same code without templates
• same code without templates and with anonymous namespaces

Answer 1

The name Foo is looked up before the lookup of Foo<Type>. The lookup for Foo is not a lookup for a base class name, so the lookup rule in [class.derived]/2 does not apply.

Refer to [basic.lookup.unqual]/7:

A name used in the definition of a class X outside of a member function body, default argument, noexcept-specifier, brace-or-equal-initializer of a non-static data member, or nested class definition25 shall be declared in one of the following ways:

• before its use in class X or be a member of a base class of X ([class.member.lookup]), or

• if X is a nested class of class Y, before the definition of X in Y, or shall be a member of a base class of Y (this lookup applies in turn to Y's enclosing classes, starting with the innermost enclosing class),26 or

• if X is a local class or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or

• if X is a member of namespace N, or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N, before the definition of class X in namespace N or in one of N's enclosing namespaces.

[ Example:

namespace M {
  class B { };
}
namespace N {
  class Y : public M::B {
    class X {
      int a[i];
    };
  };
}

// The following scopes are searched for a declaration of i:

// 1) scope of class N​::​Y​::​X, before the use of i
// 2) scope of class N​::​Y, before the definition of N​::​Y​::​X
// 3) scope of N​::​Y's base class M​::​B
// 4) scope of namespace N, before the definition of N​::​Y
// 5) global scope, before the definition of N

— end example ]

Namespace N is considered before the global namespace, so N::foo is found firstly, which causes the program ill-formed.

Answer 2

The spec says

During the lookup for a base class name, non-type names are ignored ([basic.scope.hiding])

The name is Foo<Type>, it's a type name. And the name N::Foo is not a type name, so it must be ignored. In similar situations where certain names are ignored, the wording is more explicit though

If a ​::​ scope resolution operator in a nested-name-specifier is not preceded by a decltype-specifier, lookup of the name preceding that ​::​ considers only namespaces, types, and templates whose specializations are types

Here, it doesn't only say "type names" or "non-type names" when it wants to allow type-template<arguments>. But it specifically says "templates whose specializations are types". I think this confusion is the reason why there's implementation divergence here. The name Foo<Type> is what I would call a "composite name", because it consists of nested names inside of it. So it may be unclear which exact names in it are to be ignored and which not.