Special behavior for decltype of call operator for incomplete types

I've been struggling with a compilation issue, and have been able to shrink the problem down to a small code segment.

To set the stage, I'm trying to do CRTP, where the base method calls another in the derived class. The complication is, I want to use trailing return types to get the type of forwarding directly to the the Derived class's method. This always fails to compile unless I forward to the call operator in the derived class.

This compiles:

#include <utility>

struct Incomplete;

template <typename Blah>
struct Base
{
    template <typename... Args>
    auto entry(Args&&... args)
        -> decltype(std::declval<Blah&>()(std::declval<Args&&>()...));
};

void example()
{
    Base<Incomplete> derived;
}

While this does not: (note comment for the only difference)

#include <utility>

struct Incomplete;

template <typename Blah>
struct Base
{
    template <typename... Args>
    auto entry(Args&&... args)
        -> decltype(std::declval<Blah&>().operator()(std::declval<Args&&>()...));
        //             I only added this ^^^^^^^^^^^
};

void example()
{
    Base<Incomplete> derived;
}

The error I get:

<source>: In instantiation of 'struct Base<Incomplete>':
15 : <source>:15:22:   required from here
10 : <source>:10:58: error: invalid use of incomplete type 'struct Incomplete'
         -> decltype(std::declval<Blah&>().operator()(std::declval<Args&&>()...));
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^

There seems to be some special behavior going on during resolution of the decltype in the Derived class. Is there something in the standard that would explain this?

EDIT: Made an even bigger simplification

PS: compiling example on godbolt: https://godbolt.org/g/St2gYC