[Clang][Parse] Fix ambiguity with nested-name-specifiers that may declarative (#96364)

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).

1 files changed, 2 insertions, 39 deletions

diff --git a/clang/lib/Parse/ParseCXXInlineMethods.cpp b/clang/lib/Parse/ParseCXXInlineMethods.cpp
index 9ccbbf9..b461743 100644
--- a/clang/lib/Parse/ParseCXXInlineMethods.cpp
+++ b/clang/lib/Parse/ParseCXXInlineMethods.cpp
@@ -1205,41 +1205,6 @@ bool Parser::ConsumeAndStoreConditional(CachedTokens &Toks) {
   return true;
 }
 
-/// A tentative parsing action that can also revert token annotations.
-class Parser::UnannotatedTentativeParsingAction : public TentativeParsingAction {
-public:
-  explicit UnannotatedTentativeParsingAction(Parser &Self,
-                                             tok::TokenKind EndKind)
-      : TentativeParsingAction(Self), Self(Self), EndKind(EndKind) {
-    // Stash away the old token stream, so we can restore it once the
-    // tentative parse is complete.
-    TentativeParsingAction Inner(Self);
-    Self.ConsumeAndStoreUntil(EndKind, Toks, true, /*ConsumeFinalToken*/false);
-    Inner.Revert();
-  }
-
-  void RevertAnnotations() {
-    Revert();
-
-    // Put back the original tokens.
-    Self.SkipUntil(EndKind, StopAtSemi | StopBeforeMatch);
-    if (Toks.size()) {
-      auto Buffer = std::make_unique<Token[]>(Toks.size());
-      std::copy(Toks.begin() + 1, Toks.end(), Buffer.get());
-      Buffer[Toks.size() - 1] = Self.Tok;
-      Self.PP.EnterTokenStream(std::move(Buffer), Toks.size(), true,
-                               /*IsReinject*/ true);
-
-      Self.Tok = Toks.front();
-    }
-  }
-
-private:
-  Parser &Self;
-  CachedTokens Toks;
-  tok::TokenKind EndKind;
-};
-
 /// ConsumeAndStoreInitializer - Consume and store the token at the passed token
 /// container until the end of the current initializer expression (either a
 /// default argument or an in-class initializer for a non-static data member).
@@ -1277,9 +1242,7 @@ bool Parser::ConsumeAndStoreInitializer(CachedTokens &Toks,
       //    syntactically-valid init-declarator-list, then this comma ends
       //    the default initializer.
       {
-        UnannotatedTentativeParsingAction PA(*this,
-                                             CIK == CIK_DefaultInitializer
-                                               ? tok::semi : tok::r_paren);
+        TentativeParsingAction TPA(*this, /*Unannotated=*/true);
         Sema::TentativeAnalysisScope Scope(Actions);
 
         TPResult Result = TPResult::Error;
@@ -1307,7 +1270,7 @@ bool Parser::ConsumeAndStoreInitializer(CachedTokens &Toks,
         // Put the token stream back and undo any annotations we performed
         // after the comma. They may reflect a different parse than the one
         // we will actually perform at the end of the class.
-        PA.RevertAnnotations();
+        TPA.Revert();
 
         // If what follows could be a declaration, it is a declaration.
         if (Result != TPResult::False && Result != TPResult::Error)