[clang] fix transformation of template arguments of 'auto' type constraints

See PR48617.

When assigning the new template arguments to the new TypeLoc, we were looping
on the argument count of the original TypeLoc instead of the new one,
which can be different when packs are present.

Signed-off-by: Matheus Izvekov <[email protected]>

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D109406

Author: mizvekov

clang/lib/Sema/TreeTransform.h

@@ -6578,7 +6578,7 @@ QualType TreeTransform<Derived>::TransformAutoType(TypeLocBuilder &TLB,
   NewTL.setFoundDecl(TL.getFoundDecl());
   NewTL.setLAngleLoc(TL.getLAngleLoc());
   NewTL.setRAngleLoc(TL.getRAngleLoc());
-  for (unsigned I = 0; I < TL.getNumArgs(); ++I)
+  for (unsigned I = 0; I < NewTL.getNumArgs(); ++I)
     NewTL.setArgLocInfo(I, NewTemplateArgs.arguments()[I].getLocInfo());
 
   return Result;

clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp

@@ -76,3 +76,25 @@ namespace PR48593 {
   template<class> concept d = true;
   d<,> auto e = 0; // expected-error{{expected expression}}
 }
+
+namespace PR48617 {
+  template <typename...> concept C = true;
+  template <typename...> class A {};
+
+  template <typename... Ts> C<Ts...> auto e(A<Ts...>) { return 0; }
+
+  // FIXME: The error here does not make sense.
+  template auto e<>(A<>);
+  // expected-error@-1 {{explicit instantiation of 'e' does not refer to a function template}}
+  // expected-note@-5  {{candidate template ignored: failed template argument deduction}}
+
+  // FIXME: Should be able to instantiate this with no errors.
+  template C<int> auto e<int>(A<int>);
+  // expected-error@-1 {{explicit instantiation of 'e' does not refer to a function template}}
+  // expected-note@-10 {{candidate template ignored: could not match 'C<int, Ts...> auto' against 'C<int> auto'}}
+  
+  template C<> auto e<>(A<>);
+
+  template <typename... Ts> A<Ts...> c(Ts...);
+  int f = e(c(1, 2));
+}