[clang] [clang][Preprocessor] Add peekNextPPToken, makes look ahead next token without side-effects (PR #143898)

[Erich Keane via cfe-commits]

================
@@ -2302,10 +2302,41 @@ class Preprocessor {
     }
   }
 
-  /// Determine whether the next preprocessor token to be
-  /// lexed is a '('.  If so, consume the token and return true, if not, this
+  /// Check whether the next pp-token is one of the specificed token kind. this
   /// method should have no observable side-effect on the lexed tokens.
-  bool isNextPPTokenLParen();
+  template <tok::TokenKind K, tok::TokenKind... Ks> bool isNextPPTokenOneOf() {
+    // Do some quick tests for rejection cases.
+    std::optional<Token> Val;
+    if (CurLexer)
+      Val = CurLexer->peekNextPPToken();
+    else
+      Val = CurTokenLexer->peekNextPPToken();
+
+    if (!Val) {
+      // We have run off the end.  If it's a source file we don't
+      // examine enclosing ones (C99 5.1.1.2p4).  Otherwise walk up the
+      // macro stack.
+      if (CurPPLexer)
+        return false;
+      for (const IncludeStackInfo &Entry : llvm::reverse(IncludeMacroStack)) {
+        if (Entry.TheLexer)
+          Val = Entry.TheLexer->peekNextPPToken();
+        else
+          Val = Entry.TheTokenLexer->peekNextPPToken();
+
+        if (Val)
+          break;
+
+        // Ran off the end of a source file?
+        if (Entry.ThePPLexer)
+          return false;
+      }
+    }
+
+    // Okay, we found the token and return.  Otherwise we found the end of the
+    // translation unit.
+    return Val->is(K) || (... || Val->is(Ks));
----------------
erichkeane wrote:

I woudl have suggested:
`return Val->isOneOf(K, Ks...);` perhaps?  Though perhaps I missed why this wasn't done.

https://github.com/llvm/llvm-project/pull/143898