[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

4 files changed, 112 insertions, 10 deletions

diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 8ed12d72b40..2a367bb29aa 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -6020,6 +6020,13 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                     options::OPT_fno_assume_sane_operator_new))
     CmdArgs.push_back("-fno-assume-sane-operator-new");
 
+  // -frelaxed-template-template-args is off by default, as it is a severe
+  // breaking change until a corresponding change to template partial ordering
+  // is provided.
+  if (Args.hasFlag(options::OPT_frelaxed_template_template_args,
+                   options::OPT_fno_relaxed_template_template_args, false))
+    CmdArgs.push_back("-frelaxed-template-template-args");
+
   // -fsized-deallocation is off by default, as it is an ABI-breaking change for
   // most platforms.
   if (Args.hasFlag(options::OPT_fsized_deallocation,
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index ae4417c93c2..a0682e26e70 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1960,6 +1960,8 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   if (!Opts.NoBuiltin)
     getAllNoBuiltinFuncValues(Args, Opts.NoBuiltinFuncs);
   Opts.NoMathBuiltin = Args.hasArg(OPT_fno_math_builtin);
+  Opts.RelaxedTemplateTemplateArgs =
+      Args.hasArg(OPT_frelaxed_template_template_args);
   Opts.SizedDeallocation = Args.hasArg(OPT_fsized_deallocation);
   Opts.AlignedAllocation =
       Args.hasFlag(OPT_faligned_allocation, OPT_fno_aligned_allocation,
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index ec2f5988145..facc5d1b375 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -5585,6 +5585,10 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
   return Arg;
 }
 
+static void DiagnoseTemplateParameterListArityMismatch(
+    Sema &S, TemplateParameterList *New, TemplateParameterList *Old,
+    Sema::TemplateParameterListEqualKind Kind, SourceLocation TemplateArgLoc);
+
 /// \brief Check a template argument against its corresponding
 /// template template parameter.
 ///
@@ -5601,6 +5605,9 @@ bool Sema::CheckTemplateArgument(TemplateTemplateParmDecl *Param,
     return false;
   }
 
+  if (Template->isInvalidDecl())
+    return true;
+
   // C++0x [temp.arg.template]p1:
   //   A template-argument for a template template-parameter shall be
   //   the name of a class template or an alias template, expressed as an
@@ -5628,6 +5635,25 @@ bool Sema::CheckTemplateArgument(TemplateTemplateParmDecl *Param,
   if (Param->isExpandedParameterPack())
     Params = Param->getExpansionTemplateParameters(ArgumentPackIndex);
 
+  // C++1z [temp.arg.template]p3: (DR 150)
+  //   A template-argument matches a template template-parameter P when P
+  //   is at least as specialized as the template-argument A.
+  if (getLangOpts().RelaxedTemplateTemplateArgs) {
+    // Quick check for the common case:
+    //   If P contains a parameter pack, then A [...] matches P if each of A's
+    //   template parameters matches the corresponding template parameter in
+    //   the template-parameter-list of P.
+    if (TemplateParameterListsAreEqual(
+            Template->getTemplateParameters(), Params, false,
+            TPL_TemplateTemplateArgumentMatch, Arg.getLocation()))
+      return false;
+
+    if (isTemplateTemplateParameterAtLeastAsSpecializedAs(Params, Template,
+                                                          Arg.getLocation()))
+      return false;
+    // FIXME: Produce better diagnostics for deduction failures.
+  }
+
   return !TemplateParameterListsAreEqual(Template->getTemplateParameters(),
                                          Params,
                                          true,
@@ -5839,7 +5865,7 @@ static bool MatchTemplateParameterKind(Sema &S, NamedDecl *New, NamedDecl *Old,
     return false;
   }
 
-  // Check that both are parameter packs are neither are parameter packs.
+  // Check that both are parameter packs or neither are parameter packs.
   // However, if we are matching a template template argument to a
   // template template parameter, the template template parameter can have
   // a parameter pack where the template template argument does not.
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index df796072f59..68b9853bdf6 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -1898,11 +1898,11 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
         return NumberOfArgumentsMustMatch ? Sema::TDK_TooFewArguments
                                           : Sema::TDK_Success;
 
-      if (Args[ArgIdx].isPackExpansion()) {
-        // FIXME: We follow the logic of C++0x [temp.deduct.type]p22 here,
-        // but applied to pack expansions that are template arguments.
+      // C++1z [temp.deduct.type]p9:
+      //   During partial ordering, if Ai was originally a pack expansion [and]
+      //   Pi is not a pack expansion, template argument deduction fails.
+      if (Args[ArgIdx].isPackExpansion())
         return Sema::TDK_MiscellaneousDeductionFailure;
-      }
 
       // Perform deduction for this Pi/Ai pair.
       if (Sema::TemplateDeductionResult Result
@@ -1965,7 +1965,8 @@ DeduceTemplateArguments(Sema &S,
                         TemplateDeductionInfo &Info,
                         SmallVectorImpl<DeducedTemplateArgument> &Deduced) {
   return DeduceTemplateArguments(S, TemplateParams, ParamList.asArray(),
-                                 ArgList.asArray(), Info, Deduced, false);
+                                 ArgList.asArray(), Info, Deduced,
+                                 /*NumberOfArgumentsMustMatch*/false);
 }
 
 /// \brief Determine whether two template arguments are the same.
@@ -4581,13 +4582,13 @@ UnresolvedSetIterator Sema::getMostSpecialized(
 /// Determine whether one partial specialization, P1, is at least as
 /// specialized than another, P2.
 ///
-/// \tparam PartialSpecializationDecl The kind of P2, which must be a
-/// {Class,Var}Template{PartialSpecialization,}Decl.
+/// \tparam TemplateLikeDecl The kind of P2, which must be a
+/// TemplateDecl or {Class,Var}TemplatePartialSpecializationDecl.
 /// \param T1 The injected-class-name of P1 (faked for a variable template).
 /// \param T2 The injected-class-name of P2 (faked for a variable template).
-template<typename PartialSpecializationDecl>
+template<typename TemplateLikeDecl>
 static bool isAtLeastAsSpecializedAs(Sema &S, QualType T1, QualType T2,
-                                     PartialSpecializationDecl *P2,
+                                     TemplateLikeDecl *P2,
                                      TemplateDeductionInfo &Info) {
   // C++ [temp.class.order]p1:
   //   For two class template partial specializations, the first is at least as
@@ -4729,6 +4730,72 @@ bool Sema::isMoreSpecializedThanPrimary(
   return true;
 }
 
+bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
+     TemplateParameterList *P, TemplateDecl *AArg, SourceLocation Loc) {
+  // C++1z [temp.arg.template]p4: (DR 150)
+  //   A template template-parameter P is at least as specialized as a
+  //   template template-argument A if, given the following rewrite to two
+  //   function templates...
+
+  // Rather than synthesize function templates, we merely perform the
+  // equivalent partial ordering by performing deduction directly on
+  // the template parameter lists of the template template parameters.
+  //
+  //   Given an invented class template X with the template parameter list of
+  //   A (including default arguments):
+  TemplateName X = Context.getCanonicalTemplateName(TemplateName(AArg));
+  TemplateParameterList *A = AArg->getTemplateParameters();
+
+  //    - Each function template has a single function parameter whose type is
+  //      a specialization of X with template arguments corresponding to the
+  //      template parameters from the respective function template
+  SmallVector<TemplateArgument, 8> AArgs;
+  Context.getInjectedTemplateArgs(A, AArgs);
+
+  // Check P's arguments against A's parameter list. This will fill in default
+  // template arguments as needed. AArgs are already correct by construction.
+  // We can't just use CheckTemplateIdType because that will expand alias
+  // templates.
+  SmallVector<TemplateArgument, 4> PArgs;
+  {
+    SFINAETrap Trap(*this);
+
+    Context.getInjectedTemplateArgs(P, PArgs);
+    TemplateArgumentListInfo PArgList(P->getLAngleLoc(), P->getRAngleLoc());
+    for (unsigned I = 0, N = P->size(); I != N; ++I) {
+      // Unwrap packs that getInjectedTemplateArgs wrapped around pack
+      // expansions, to form an "as written" argument list.
+      TemplateArgument Arg = PArgs[I];
+      if (Arg.getKind() == TemplateArgument::Pack) {
+        assert(Arg.pack_size() == 1 && Arg.pack_begin()->isPackExpansion());
+        Arg = *Arg.pack_begin();
+      }
+      PArgList.addArgument(getTrivialTemplateArgumentLoc(
+          Arg, QualType(), P->getParam(I)->getLocation()));
+    }
+    PArgs.clear();
+
+    // C++1z [temp.arg.template]p3:
+    //   If the rewrite produces an invalid type, then P is not at least as
+    //   specialized as A.
+    if (CheckTemplateArgumentList(AArg, Loc, PArgList, false, PArgs) ||
+        Trap.hasErrorOccurred())
+      return false;
+  }
+
+  QualType AType = Context.getTemplateSpecializationType(X, AArgs);
+  QualType PType = Context.getTemplateSpecializationType(X, PArgs);
+
+  SmallVector<DeducedTemplateArgument, 4> Deduced;
+  Deduced.resize(A->size());
+
+  //   ... the function template corresponding to P is at least as specialized
+  //   as the function template corresponding to A according to the partial
+  //   ordering rules for function templates.
+  TemplateDeductionInfo Info(Loc, A->getDepth());
+  return isAtLeastAsSpecializedAs(*this, PType, AType, AArg, Info);
+}
+
 static void
 MarkUsedTemplateParameters(ASTContext &Ctx,
                            const TemplateArgument &TemplateArg,