The following piece of code is a reduced sample from the large project I’m

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Asked: May 16, 20262026-05-16T04:13:33+00:00 2026-05-16T04:13:33+00:00

The following piece of code is a reduced sample from the large project I’m

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The following piece of code is a reduced sample from the large project I’m trying to port from GCC/G++ to Microsoft Visual C++ 2010. It compiles fine with G++, but with MSVC++, it throws errors, and I’m having trouble understanding why.

template <typename A, typename B = typename A::C::D>    // line 1
struct foo
{
  typedef int type;
};

template <template <typename> class E, typename T>
typename foo<E<T> >::type                               // line 8
bar(){}

The error messages from MSVC++ are:

example1.cpp(1) : error C2027: use of undefined type 'E<T>'
example1.cpp(8) : error C2146: syntax error : missing ',' before identifier 'D'
example1.cpp(8) : error C2065: 'D' : undeclared identifier

I’ve tried a few changes to narrow down the problem a bit, and while I don’t fully understand it, here’s what I’ve discovered: If in line 1 I replace A::C::D with A::C, it works fine. If I replace template <typename> class E with just typename E and make that foo<E>, it works fine. If explicitly specify the second template argument to foo in line 8, like so, it works fine:

typename foo<E<T>, typename E<T>::C::D>::type          // line 8

And, if I replace the use of A::C::D with something innocuous like typename B = A in line 1, but add a different use of A::C::D as typedef typename A::C::D qux; to the definition of foo, that also works fine.

Any ideas? What bit of C++ rules am I missing?