Is this a bug in GCC?

Question

EDIT: This is not a bug, just me not knowing about dependent name lookups in templated base classes (which MSVC \"helpfully\" resolves without errors).

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I wrote

Answer 1

This is a bug in MSVC instead. Names from dependent base classes have to be "thisambiguated".

The reason is that unqualified lookup of dependent names proceeds in two phases. During the first phase, the base class is not yet known and the compiler cannot resolve the name. MSVC does not implement two-phase name lookup and delays the lookup until the second phase.

The full specialization

template<>
struct Event<void, void> : public EventBase<Event<> >
{
        void trigger() const
        {
                // No error here even without `this`, for some reason!
                auto it = subscribers.cbegin();
        }
};

does not suffer from this problem, because both the class and its base are regular classes, not class templates, and there is no template dependency to begin with.

When porting C++ code from MSVC to gcc/Clang, dependent name lookup disambiguation and the template keyword disambiguation (i.e. calling member function template using ::template, ->template or .template syntax) are two of the subtleties that you have to deal with (empty base optimization is another one). For all the Standards compliance rhetoric, this will probably never be fixed for reasons of backwards compatibility.