[Clang] Fix dependency computation for pack indexing expression (#91933)

Given `foo...[idx]` if idx is value dependent, the expression is type
dependent.

Fixes #91885
Fixes #91884

Author: cor3ntin

clang/lib/AST/ComputeDependence.cpp

@@ -375,12 +375,19 @@ ExprDependence clang::computeDependence(PackExpansionExpr *E) {
 }
 
 ExprDependence clang::computeDependence(PackIndexingExpr *E) {
+
+  ExprDependence PatternDep = E->getPackIdExpression()->getDependence() &
+                              ~ExprDependence::UnexpandedPack;
+
   ExprDependence D = E->getIndexExpr()->getDependence();
+  if (D & ExprDependence::TypeValueInstantiation)
+    D |= E->getIndexExpr()->getDependence() | PatternDep |
+         ExprDependence::Instantiation;
+
   ArrayRef<Expr *> Exprs = E->getExpressions();
   if (Exprs.empty())
-    D |= (E->getPackIdExpression()->getDependence() |
-          ExprDependence::TypeValueInstantiation) &
-         ~ExprDependence::UnexpandedPack;
+    D |= PatternDep | ExprDependence::Instantiation;
+
   else if (!E->getIndexExpr()->isInstantiationDependent()) {
     std::optional<unsigned> Index = E->getSelectedIndex();
     assert(Index && *Index < Exprs.size() && "pack index out of bound");

clang/lib/Sema/SemaType.cpp

@@ -9351,15 +9351,20 @@ QualType Sema::BuildCountAttributedArrayType(QualType WrappedTy,
 /// that expression, according to the rules in C++11
 /// [dcl.type.simple]p4 and C++11 [expr.lambda.prim]p18.
 QualType Sema::getDecltypeForExpr(Expr *E) {
-  if (E->isTypeDependent())
-    return Context.DependentTy;
 
   Expr *IDExpr = E;
   if (auto *ImplCastExpr = dyn_cast<ImplicitCastExpr>(E))
     IDExpr = ImplCastExpr->getSubExpr();
 
-  if (auto *PackExpr = dyn_cast<PackIndexingExpr>(E))
-    IDExpr = PackExpr->getSelectedExpr();
+  if (auto *PackExpr = dyn_cast<PackIndexingExpr>(E)) {
+    if (E->isInstantiationDependent())
+      IDExpr = PackExpr->getPackIdExpression();
+    else
+      IDExpr = PackExpr->getSelectedExpr();
+  }
+
+  if (E->isTypeDependent())
+    return Context.DependentTy;
 
   // C++11 [dcl.type.simple]p4:
   //   The type denoted by decltype(e) is defined as follows:

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

@@ -194,3 +194,26 @@ void h() {
   // expected-note-re@-2 {{function template specialization '{{.*}}' requested here}}
 }
 }
+
+namespace GH91885 {
+
+void test(auto...args){
+    [&]<int idx>(){
+        using R = decltype( args...[idx] ) ;
+    }.template operator()<0>();
+}
+
+template<int... args>
+void test2(){
+  [&]<int idx>(){
+    using R = decltype( args...[idx] ) ;
+  }.template operator()<0>();
+}
+
+void f( ) {
+  test(1);
+  test2<1>();
+}
+
+
+}