[clang] Allow pack expansions when partial ordering against template template parameters

clang/docs/ReleaseNotes.rst

@@ -708,6 +708,8 @@ Bug Fixes to C++ Support
   expression.
 - Fix a bug in access control checking due to dealyed checking of friend declaration. Fixes (#GH12361).
 - Correctly treat the compound statement of an ``if consteval`` as an immediate context. Fixes (#GH91509).
+- When partial ordering alias templates against template template parameters,
+  allow pack expansions when the alias has a fixed-size parameter list. Fixes (#GH62529).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -9216,14 +9216,20 @@ class Sema final : public SemaBase {
   /// receive true if the cause for the error is the associated constraints of
   /// the template not being satisfied by the template arguments.
   ///
+  /// \param PartialOrderingTTP If true, assume these template arguments are
+  /// the injected template arguments for a template template parameter.
+  /// This will relax the requirement that all its possible uses are valid:
+  /// TTP checking is loose, and assumes that invalid uses will be diagnosed
+  /// during instantiation.
+  ///
   /// \returns true if an error occurred, false otherwise.
   bool CheckTemplateArgumentList(
       TemplateDecl *Template, SourceLocation TemplateLoc,
       TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
       SmallVectorImpl<TemplateArgument> &SugaredConverted,
       SmallVectorImpl<TemplateArgument> &CanonicalConverted,
       bool UpdateArgsWithConversions = true,
-      bool *ConstraintsNotSatisfied = nullptr);
+      bool *ConstraintsNotSatisfied = nullptr, bool PartialOrderingTTP = false);
 
   bool CheckTemplateTypeArgument(
       TemplateTypeParmDecl *Param, TemplateArgumentLoc &Arg,

clang/lib/Sema/SemaTemplate.cpp

@@ -6556,7 +6556,8 @@ bool Sema::CheckTemplateArgumentList(
     TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
     SmallVectorImpl<TemplateArgument> &SugaredConverted,
     SmallVectorImpl<TemplateArgument> &CanonicalConverted,
-    bool UpdateArgsWithConversions, bool *ConstraintsNotSatisfied) {
+    bool UpdateArgsWithConversions, bool *ConstraintsNotSatisfied,
+    bool PartialOrderingTTP) {
 
   if (ConstraintsNotSatisfied)
     *ConstraintsNotSatisfied = false;
@@ -6627,9 +6628,14 @@ bool Sema::CheckTemplateArgumentList(
       bool PackExpansionIntoNonPack =
           NewArgs[ArgIdx].getArgument().isPackExpansion() &&
           (!(*Param)->isTemplateParameterPack() || getExpandedPackSize(*Param));
-      if (PackExpansionIntoNonPack && (isa<TypeAliasTemplateDecl>(Template) ||
-                                       isa<ConceptDecl>(Template))) {
-        // Core issue 1430: we have a pack expansion as an argument to an
+      // CWG1430: Don't diagnose this pack expansion when partial
+      // ordering template template parameters. Some uses of the template could
+      // be valid, and invalid uses will be diagnosed later during
+      // instantiation.
+      if (PackExpansionIntoNonPack && !PartialOrderingTTP &&
+          (isa<TypeAliasTemplateDecl>(Template) ||
+           isa<ConceptDecl>(Template))) {
+        // CWG1430: we have a pack expansion as an argument to an
         // alias template, and it's not part of a parameter pack. This
         // can't be canonicalized, so reject it now.
         // As for concepts - we cannot normalize constraints where this

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -6243,7 +6243,9 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
     //   specialized as A.
     SmallVector<TemplateArgument, 4> SugaredPArgs;
     if (CheckTemplateArgumentList(AArg, Loc, PArgList, false, SugaredPArgs,
-                                  PArgs) ||
+                                  PArgs, /*UpdateArgsWithConversions=*/true,
+                                  /*ConstraintsNotSatisfied=*/nullptr,
+                                  /*PartialOrderTTP=*/true) ||
         Trap.hasErrorOccurred())
       return false;
   }

clang/test/SemaTemplate/temp_arg_template_cxx1z.cpp → clang/test/SemaTemplate/temp_arg_template_p0522.cpp

@@ -1,6 +1,6 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 
-// expected-note@temp_arg_template_cxx1z.cpp:* 1+{{}}
+// expected-note@temp_arg_template_p0522.cpp:* 1+{{}}
 
 template<template<int> typename> struct Ti;
 template<template<int...> typename> struct TPi;
@@ -118,3 +118,11 @@ namespace Auto {
   TInt<SubstFailure> isf; // FIXME: this should be ill-formed
   TIntPtr<SubstFailure> ipsf;
 }
+
+namespace GH62529 {
+  // Note: the constraint here is just for bypassing a fast-path.
+  template<class T1> requires(true) using A = int;
+  template<template<class ...T2s> class TT1, class T3> struct B {};
+  template<class T4> B<A, T4> f();
+  auto t = f<int>();
+} // namespace GH62529