[Concepts] Implement P1616R1 - Using unconstrained template template parameters with constrained templates

[saar.raz]

Summary

Allow unconstrained template template parameters to accept constrainted templates as arguments.

clang/lib/Sema/SemaTemplate.cpp

if (getLangOpts().RelaxedTemplateTemplateArgs) {
     if (isTemplateTemplateParameterAtLeastAsSpecializedAs(Params, Template,
                                                           Arg.getLocation())) {
       // C++2a[temp.func.order]p2
       //   [...] If both deductions succeed, the partial ordering selects the
       //   more constrained template as described by the rules in
       //   [temp.constr.order].
       SmallVector<const Expr *, 3> ParamsAC, TemplateAC;
       Params->getAssociatedConstraints(ParamsAC);
+      // C++2a[temp.arg.template]p3
+      //   [...] In this comparison, if P is unconstrained, the constraints on A
+      //   are not considered.
+      if (ParamsAC.empty())
+        return false;
       Template->getAssociatedConstraints(TemplateAC);
       bool IsParamAtLeastAsConstrained;
       if (IsAtLeastAsConstrained(Param, ParamsAC, Template, TemplateAC,
                                  IsParamAtLeastAsConstrained))
         return true;
       if (!IsParamAtLeastAsConstrained) {
         Diag(Arg.getLocation(),
              diag::err_template_template_parameter_not_at_least_as_constrained)

clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp

 // RUN:  %clang_cc1 -std=c++2a -fconcepts-ts -frelaxed-template-template-args -verify %s
 
 template<typename T> concept C = T::f();
 // expected-note@-1{{similar constraint}}
 template<typename T> concept D = C<T> && T::g();
 template<typename T> concept F = T::f();
 // expected-note@-1{{similar constraint expressions not considered equivalent}}
 template<template<C> class P> struct S1 { }; // expected-note 2{{'P' declared here}}
 
-template<C> struct X { }; // expected-note{{'X' declared here}}
+template<C> struct X { };
 
-template<D> struct Y { }; // expected-note 2{{'Y' declared here}}
+template<D> struct Y { }; // expected-note{{'Y' declared here}}
 template<typename T> struct Z { };
 template<F> struct W { }; // expected-note{{'W' declared here}}
 
 S1<X> s11;
 S1<Y> s12; // expected-error{{template template argument 'Y' is more constrained than template template parameter 'P'}}
 S1<Z> s13;
 S1<W> s14; // expected-error{{template template argument 'W' is more constrained than template template parameter 'P'}}
 
-template<template<typename> class P> struct S2 { }; // expected-note 2{{'P' declared here}}
+template<template<typename> class P> struct S2 { };
 
-S2<X> s21; // expected-error{{template template argument 'X' is more constrained than template template parameter 'P'}}
-S2<Y> s22; // expected-error{{template template argument 'Y' is more constrained than template template parameter 'P'}}
+S2<X> s21;
+S2<Y> s22;
 S2<Z> s23;
 
 template <template <typename...> class C>
 struct S3;
 
 template <C T>
 using N = typename T::type;
 
 using s31 = S3<N>;
 using s32 = S3<Z>;