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708a9a06cb
Consider the following:
```
template<typename T>
struct A { };
template<typename T>
int A<T>::B::* f(); // error: no member named 'B' in 'A<T>'
```
Although this is clearly valid, clang rejects it because the
_nested-name-specifier_ `A<T>::` is parsed as-if it was declarative,
meaning, we parse it as-if it was the _nested-name-specifier_ in a
redeclaration/specialization. However, we don't (and can't) know whether
the _nested-name-specifier_ is declarative until we see the '`*`' token,
but at that point we have already complained that `A` has no member
named `B`! This patch addresses this bug by adding support for _fully_
unannotated _and_ unbounded tentative parsing, which allows for us to
parse past tokens without having to cache them until we reach a point
where we can guarantee to be past the construct we are disambiguating.
I don't know where the approach taken here is ideal -- alternatives are
welcome. However, the performance impact (as measured by
llvm-compile-time-tracker (https://llvm-compile-time-tracker.com/?config=Overview&stat=instructions%3Au&remote=sdkrystian)
is quite minimal (0.09%, which I plan to further improve).