Always deduce the lengths of contained parameter packs when deducing a

pack expansion.

Previously, if all parameter / argument pairs for a pack expansion
deduction were non-deduced contexts, we would not deduce the arity of
the pack, and could end up deducing a different arity (leading to
failures during substitution) or defaulting to an arity of 0 (leading to
bad diagnostics about passing the wrong number of arguments to a
variadic function). Instead, we now always deduce the arity for all
involved packs any time we deduce a pack expansion.

This will result in less substitution happening in some cases, which
could avoid non-SFINAEable errors, and should generally improve the
quality of diagnostics when passing initializer lists to variadic
functions.

1 files changed, 2 insertions, 4 deletions

diff --git a/clang/test/SemaTemplate/alias-templates.cpp b/clang/test/SemaTemplate/alias-templates.cpp
index 240a6eeff2e..80678bf2298 100644
--- a/clang/test/SemaTemplate/alias-templates.cpp
+++ b/clang/test/SemaTemplate/alias-templates.cpp
@@ -77,14 +77,12 @@ namespace PR11848 {
     return i + f1<Ts...>(is...);
   }
 
-  // FIXME: This note is technically correct, but could be better. We
-  // should really say that we couldn't infer template argument 'Ts'.
   template<typename ...Ts>
-  void f2(U<Ts> ...is) { } // expected-note {{requires 0 arguments, but 1 was provided}}
+  void f2(U<Ts> ...is) { } // expected-note {{deduced incomplete pack <(no value)> for template parameter 'Ts'}}
 
   template<typename...> struct type_tuple {};
   template<typename ...Ts>
-  void f3(type_tuple<Ts...>, U<Ts> ...is) {} // expected-note {{requires 4 arguments, but 3 were provided}}
+  void f3(type_tuple<Ts...>, U<Ts> ...is) {} // expected-note {{deduced packs of different lengths for parameter 'Ts' (<void, void, void> vs. <(no value), (no value)>)}}
 
   void g() {
     f1(U<void>()); // expected-error {{no match}}