[Clang] Do not try to diagnose parameter packs in invalid pack expressions (#89257)

In a pack expression, if the id-expression is not valid, do no try to
detect whether it is a pack as that would lead to a crash trying to
print a recovery expression.

Fixes #88929

Author: cor3ntin

clang/lib/Sema/SemaTemplateVariadic.cpp

@@ -1085,9 +1085,11 @@ ExprResult Sema::ActOnPackIndexingExpr(Scope *S, Expr *PackExpression,
                                        SourceLocation RSquareLoc) {
   bool isParameterPack = ::isParameterPack(PackExpression);
   if (!isParameterPack) {
-    CorrectDelayedTyposInExpr(IndexExpr);
-    Diag(PackExpression->getBeginLoc(), diag::err_expected_name_of_pack)
-        << PackExpression;
+    if (!PackExpression->containsErrors()) {
+      CorrectDelayedTyposInExpr(IndexExpr);
+      Diag(PackExpression->getBeginLoc(), diag::err_expected_name_of_pack)
+          << PackExpression;
+    }
     return ExprError();
   }
   ExprResult Res =

clang/test/SemaCXX/cxx2c-pack-indexing.cpp

@@ -154,3 +154,9 @@ void f() {
 }
 
 }
+
+namespace GH88929 {
+    bool b = a...[0];  // expected-error {{use of undeclared identifier 'a'}}
+    using E = P...[0]; // expected-error {{unknown type name 'P'}} \
+                       // expected-error {{expected ';' after alias declaration}}
+}