Why do we need typename here?

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[*]

In general, C++ needs typename because of the unfortunate syntax [*] it inherits from C, that makes it impossible without non-local information to say — for example — in A * B; whether A names a type (in which case this is a declaration of B as a pointer to it) or not (in which case this is a multiplication expression — quite possible since A, for all you can tell without non-local information, could be an instance of a class that overloads operator* to do something weird;-).

In most cases the compiler does have the non-local information needed to disambiguate (though the unfortunate syntax still means the low-level parser needs feedback from the higher-level layer that keeps the symbol table info)… but with templates it doesn’t (not in general, though in this specific case it might be technically illegal to specialize a std::list<T> so that its ::iterator is NOT a type name;-).

[*] not just my opinion, but also the opinion of Ken Thompson and Rob Pikes, currently my colleagues, who are busy designing and implementing a new programming language for internal use: that new programming language, while its syntax is mostly C-like, does NOT repeat C’s syntax design errors — it the new language (like e.g. in good old Pascal), syntax is sufficient to distinguish identifiers that must name a type from ones that must not;-).

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