forward reference to class template - template param not in scope

I'm seeing what look like buggy results from the compiler where it chokes on the use of a T<X>& to a declared but undefined class template T (a forward reference), iff the fwd reference is to a dependent type name (e.g. X is a template parameter).

Looks rather like a 2-phase name lookup bug of some kind - an area where the compiler has had conformance issues with the C++ Standard.

Here is a file I wrote that reproduces the problem as simply as possible:

template <class OuterT>
struct Grog
{
    template <class InnerT>
    struct Foo; // forward reference

    template <class InnerT>
    struct Bar
    {
        // class template Foo<> is declared but not yet defined

        Bar(Foo<int>&); // Foo<int> is not a dependent type name; compiles OK

        Bar(Foo<InnerT>&); // Foo<InnerT> is a dependent type name
        //..fails to compile
        //..the compiler errors caused by compiling this line are reported for the line below
    };

    template <class InnerT>
    struct Foo
    {
        Foo(OuterT&)
        //..error C2143: syntax error : missing ')' before '&'
        //..    see reference to class template instantiation 'Grog<OuterT>::Foo<InnerT>' being compiled
        //..    see reference to class template instantiation 'Grog<OuterT>' being compiled
        //..error C2143: syntax error : missing ';' before '&'
        //..error C2059: syntax error : ')'
        //..error C2146: syntax error : missing ';' before identifier 'makeBar'
        //..error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
        //..error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
        //..warning C4183: 'makeBar': missing return type; assumed to be a member function returning 'int'

        // it's choking on 'OuterT' - unrecognized, although it ought to be in scope here

        Bar<InnerT> makeBar();
        // the fwd reference (above) is needed because we return an object of type Bar<> here
        // thus it would be impossible to define Foo<> before Bar<>
    };
};

This is very inconvenient -- totally debilitating in fact, for my code -- is there a work-around

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forward reference to class template - template param not in scope

coolVijay
The typedef workaround is working for me. I don't get the error everywhere I expected to, it's peculiar how localized the bug is -- I can't figure out what its modality is. But so far I only had to change one typedef (and lots of references).

Thanks much for your speedy reply! :-)

-dc
VB_Devil
I've seen something similar, but I thought it was fixed in Whidbey. I don't really see what this has to do with two-phase name lookup. Anyway, it's typically good enough to add a typedef for the template type parameter.

-hg
Kerryrob
It's not fixed (in the final release of Visual C++ 2005 Express).

BTW sorry about the missing ';' in the code I pasted -- here is the code I actually meant!

The typedef workaround does seem to work for this simple example anyway. I will let you know if I get away with it after adding (many) typedefs to my much larger real-world project.

struct Grog
{
    template <class InnerT>
    struct Foo; // forward reference

    template <class InnerT>
    struct Bar
    {
        // class template Foo<> is declared but not yet defined

        Bar(Foo<int>&); // Foo<int> is not a dependent type name; compiles OK

        Bar(Foo<InnerT>&); // Foo<InnerT> is a dependent type name
        //..fails to compile
        //..the compiler errors caused by compiling this line are reported for the line below
    };

    typedef OuterT OuterTypeSynonym;

    template <class InnerT>
    struct Foo
    {
        Foo(OuterT&);
        //..error C2143: syntax error : missing ')' before '&'
        //..    see reference to class template instantiation 'Grog<OuterT>::Foo<InnerT>' being compiled
        //..    see reference to class template instantiation 'Grog<OuterT>' being compiled
        //..error C2143: syntax error : missing ';' before '&'
        //..error C2059: syntax error : ')'

        // it's choking on 'OuterT' - unrecognized, although it ought to be in scope here
        // change OuterT to OuterTypeSynonym and it compiles
        // strangely, even when the typedef occurs after the fwd reference

        Bar<InnerT> makeBar();
        // the fwd reference (above) is needed because we return an object of type Bar<> here
        // thus it would be impossible to define Foo<> before Bar<>
    };
};
forward reference to class template - template param not in scope

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