[c++17] Implement P0522R0 as written. This allows a template template argument

to be specified for a template template parameter whenever the parameter is at
least as specialized as the argument (when there's an obvious and correct
mapping from uses of the parameter to uses of the argument). For example, a
template with more parameters can be passed to a template template parameter
with fewer, if those trailing parameters have default arguments.

This is disabled by default, despite being a DR resolution, as it's fairly
broken in its current state: there are no partial ordering rules to cope with
template template parameters that have different parameter lists, meaning that
code that attempts to decompose template-ids based on arity can hit unavoidable
ambiguity issues.

The diagnostics produced on a non-matching argument are also pretty bad right
now, but I aim to improve them in a subsequent commit.

llvm-svn: 290792

1 files changed, 3 insertions, 1 deletions

diff --git a/clang/test/SemaTemplate/temp_arg_template.cpp b/clang/test/SemaTemplate/temp_arg_template.cpp
index 6d93f1504a4..67cde53c928 100644
--- a/clang/test/SemaTemplate/temp_arg_template.cpp
+++ b/clang/test/SemaTemplate/temp_arg_template.cpp
@@ -6,11 +6,12 @@ template<template<typename T> class X> struct A; // expected-note 2{{previous te
 
 template<template<typename T, int I> class X> struct B; // expected-note{{previous template template parameter is here}}
 
-template<template<int I> class X> struct C;  // expected-note{{previous non-type template parameter with type 'int' is here}}
+template<template<int I> class X> struct C;  // expected-note 2{{previous non-type template parameter with type 'int' is here}}
 
 template<class> struct X; // expected-note{{too few template parameters in template template argument}}
 template<int N> struct Y; // expected-note{{template parameter has a different kind in template argument}}
 template<long N> struct Ylong; // expected-note{{template non-type parameter has a different type 'long' in template argument}}
+template<const int &N> struct Yref; // expected-note{{template non-type parameter has a different type 'const int &' in template argument}}
 
 namespace N {
   template<class> struct Z;
@@ -27,6 +28,7 @@ A<TooMany> *a5; // expected-error{{template template argument has different temp
 B<X> *a6; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
 C<Y> *a7;
 C<Ylong> *a8; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
+C<Yref> *a9; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
 
 template<typename T> void f(int);