From 02dece03f93d2ff847c3867e5f655793e29b2877 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov@gmail.com>
Date: Wed, 8 Sep 2021 02:21:17 +0200
Subject: [PATCH] [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 <mizvekov@gmail.com>

Reviewed By: rsmith

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

(cherry picked from commit 68b9d8ed7abe4046992ae1557990edfbb3a772bc)
---
 clang/lib/Sema/TreeTransform.h                |  2 +-
 .../dcl.spec/dcl.type/dcl.spec.auto/p6.cpp    | 22 +++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 70ba631dbfc6c5..d8a5b6ad4f948c 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/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;
diff --git a/clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp b/clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp
index 7830d1f435262b..49410274232172 100644
--- a/clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp
+++ b/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));
+}